Inflammopharmacol (2013) 21:201232

DOI 10.1007/s10787-013-0172-x Inflammopharmacology

REVIEW

The modern pharmacology of paracetamol: therapeutic actions,

mechanism of action, metabolism, toxicity and recent
pharmacological findings
Garry G. Graham Michael J. Davies
Richard O. Day Anthoulla Mohamudally

Kieran F. Scott

Received: 4 February 2013 / Accepted: 18 April 2013 / Published online: 30 May 2013
Springer Basel 2013

Abstract Paracetamol is used worldwide for its analgesic and peroxides are available but conversely, it has little
and antipyretic actions. It has a spectrum of action similar to activity at substantial levels of arachidonic acid and per-
that of NSAIDs and resembles particularly the COX-2 oxides. The result is that paracetamol does not suppress the
selective inhibitors. Paracetamol is, on average, a weaker severe inflammation of rheumatoid arthritis and acute gout
analgesic than NSAIDs or COX-2 selective inhibitors but is but does inhibit the lesser inflammation resulting from
often preferred because of its better tolerance. Despite the extraction of teeth and is also active in a variety of
similarities to NSAIDs, the mode of action of paracetamol inflammatory tests in experimental animals. Paracetamol
has been uncertain, but it is now generally accepted that it often appears to have COX-2 selectivity. The apparent
inhibits COX-1 and COX-2 through metabolism by the COX-2 selectivity of action of paracetamol is shown by its
peroxidase function of these isoenzymes. This results in poor anti-platelet activity and good gastrointestinal toler-
inhibition of phenoxyl radical formation from a critical ance. Unlike both non-selective NSAIDs and selective
tyrosine residue essential for the cyclooxygenase activity of COX-2 inhibitors, paracetamol inhibits other peroxidase
COX-1 and COX-2 and prostaglandin (PG) synthesis. Par- enzymes including myeloperoxidase. Inhibition of myelo-
acetamol shows selectivity for inhibition of the synthesis of peroxidase involves paracetamol oxidation and
PGs and related factors when low levels of arachidonic acid concomitant decreased formation of halogenating oxidants
(e.g. hypochlorous acid, hypobromous acid) that may be
associated with multiple inflammatory pathologies includ-
G. G. Graham (&)  R. O. Day
Department of Clinical Pharmacology and Toxicology, ing atherosclerosis and rheumatic diseases. Paracetamol
St Vincents Hospital, University of New South Wales, may, therefore, slow the development of these diseases.
Sydney, Australia Paracetamol, NSAIDs and selective COX-2 inhibitors all
e-mail: g.graham@unsw.edu.au
have central and peripheral effects. As is the case with the
G. G. Graham  R. O. Day NSAIDs, including the selective COX-2 inhibitors, the
Department of Pharmacology, University of New South Wales, analgesic effects of paracetamol are reduced by inhibitors of
Sydney, Australia many endogenous neurotransmitter systems including
serotonergic, opioid and cannabinoid systems. There is
M. J. Davies
Faculty of Medicine, University of Sydney, Sydney, Australia considerable debate about the hepatotoxicity of therapeutic
doses of paracetamol. Much of the toxicity may result from
M. J. Davies overuse of combinations of paracetamol with opioids which
The Heart Research Institute, Sydney, Australia
are widely used, particularly in USA.
A. Mohamudally
Territory Palliative Care, Royal Darwin Hospital, Darwin, Keywords Paracetamol  Acetaminophen  Pain 
Australia Analgesia  Inflammation  Prostaglandin  Thromboxane 
Prostacyclin  Cyclooxygenase, COX-1  COX-2 
K. F. Scott
School of Medicine, University of Western Sydney, Sydney, COX-3  Myeloperoxidase  Atherosclerosis  Peroxidase 
Australia MIF  Diabetes, rhabdomyolysis  Resveratrol

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202 G. G. Graham et al.

Introduction Fig. 1 Structure of paracetamol

Paracetamol (acetaminophen) is one of the worlds most

widely used non-prescription medicines from cradle to
grave. It is readily available and inexpensive. As an anal-
gesic, paracetamol is better tolerated than the non-steroidal
anti-inflammatory drugs (NSAIDs) although it may be
somewhat less efficacious. During the 1980s a decline in
the use of aspirin due to its association with Reyes syn-
drome allowed paracetamol to become the antipyretic and dosage (Prescott et al. 1989), it is concluded that paracet-
analgesic of choice in children (Belay et al. 1999) and it is amol distributes throughout the body without binding to
now the standard antipyretic and analgesic in all age tissues. This lack of binding indicates that the concentra-
groups. Although a useful and important drug, the dose of tions of paracetamol in experiments in vitro can be
paracetamol is inconveniently large and a full dose of 4 g correlated directly with its concentrations in vivo without
daily requires a large number of tablets to be taken. corrections for tissue uptake or protein binding. The peak
In his Nobel Prize-winning work on the mechanism of plasma concentrations of paracetamol after a therapeutic
action of aspirin and other NSAIDs, Vane (1971) demon- dose (1 g) are approximately 20 mg/L (130 lM) up to
strated that these drugs inhibit the formation of 30 mg/L (200 lM) after intravenous injection or rapid oral
prostaglandins (PGs), local factors that are associated with absorption, and concentrations of up to 30 mg/L can be
pain, fever and inflammation. However, paracetamol did considered as therapeutic. At a dosage of 1 g four times
not appear to inhibit PG synthesis, despite its actions daily, the trough concentrations are of the order of 2 mg/L
similar to those of the NSAIDs. The mechanism of the (13 lM).
basic pharmacological effects of paracetamol is only now Chemically, paracetamol is a phenol and, like many
becoming clear and it is now recognised to be an inhibitor phenols, it is easily oxidised. This oxidation is central to its
of PG synthesis in cellular systems under specific condi- postulated mechanism of action as a substrate and an
tions and has an apparent selectivity for one of the inhibitor of the peroxidase function of COX-1 and COX-2.
cyclooxygenase (COX) enzymes, namely COX-2. Paracetamol is also oxidised by and inhibits other haem
This article is a review of the pharmacology of para- peroxidases, including myeloperoxidase.
cetamol, particularly on its mechanism of action and
therapeutic effects, with an emphasis on discoveries that
have been made in the past 10 years. Some aspects of the Therapeutic and toxic actions: similarities
clinical pharmacology of paracetamol, such as its phar- and differences from NSAIDs
macokinetics, metabolism and adverse effects are not
covered in detail although its metabolism by peroxidases Much of the pharmacology and toxicology of paracetamol
and the claimed hepatotoxicity of therapeutic doses are are similar to those of the non-selective NSAIDs, such as
reviewed. New pharmacological actions of paracetamol ibuprofen, ketoprofen and naproxen, and it shows partic-
have been identified in recent years, particularly its inter- ular similarity to the selective COX-2 inhibitors, such as
action with haem peroxidases, such as myeloperoxidase, celecoxib and etoricoxib (Table 1). On average, however,
that is discussed in this review. These recently discovered paracetamol has weaker analgesic activity than both groups
actions have largely been detected in vitro but may lead to of NSAIDs (see Clinical analgesic efficacy of paraceta-
new clinical uses of this old drug. mol section). A major difference is that paracetamol,
unlike both groups of NSAIDs, has only weak anti-
inflammatory activity (Table 1; see Anti-inflammatory
Chemistry and distribution effect).
Much of the toxicity of therapeutic doses of NSAIDs,
Paracetamol is a low-molecular-mass compound (Fig. 1). It particularly that arising from the older non-selective
is an extremely weak acid (pKa 9.7) and is, therefore, NSAIDs, is not seen with paracetamol. In particular, par-
essentially unionised at physiological pH values (Craig acetamol does not cause significant gastrointestinal toxicity
1990). Its partition coefficient between octanol and water is at therapeutic doses (Zhang et al. 2004). Paracetamol is
3.2 and in the range where passive diffusion through cell also a weak precipitant of asthma in aspirin-sensitive
membranes is likely. The binding of paracetamol to plasma asthmatics although it may increase the incidence of
proteins is negligible Gazzard et al. (1973) and with a asthma (see Bronchoconstriction and asthma section).
volume of distribution of about 50 L after intravenous Paracetamol does, however, have specific and dangerous

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The modern pharmacology of paracetamol 203

Table 1 Summary of pharmacological and clinical activities of paracetamol, selective COX-2 inhibitors and non-selective NSAIDs
Pharmacological activity Paracetamol Selective COX-2 inhibitor Non-selective NSAID

Analgesia Active Active Active

Antipyresis Active Active Active
Anti-inflammatory Active in mild inflammation Active Active
Anti-platelet Low activity Inactive Active
Damage to stomach and small Low activity Low activity Active
intestine
Aspirin-induced asthma Weakly active Inactive Active
Blood pressure Variable data Increase Increase
Renal Lesser effects than both NSAID Impaired function in stressed Impaired function in stressed
classes kidneys kidneys
Increased risk of thrombosis Inactive Active Active

hepatotoxicity of a type that is not seen with the NSAIDs et al. 2003; Loboz and Shenfield 2005). In patients with
(see Adverse effects section). By contrast, apart from these risk factors, the function of PGs is important in
aspirin and salicylate, both types of NSAIDs overdoses do maintaining renal function and significant inhibition of their
not produce life-threatening reactions. synthesis leads to renal impairment.
Both classes of NSAIDs have been associated with Although paracetamol has apparent COX-2 inhibitory
increase in blood pressure (Laine et al. 2008), more so in activity (see Reasons for the apparent COX-2 selectivity
patients treated for hypertension than normotensive indi- of paracetamol section), it is widely regarded as being
viduals (Snowden and Nelson 2011). The effect of safe in patients with risk factors for renal impairment in
paracetamol has been studied to a lesser extent with contrast to patients taking NSAIDs. Experimental proof of
inconsistent results (Sudano et al. 2012; Turtle et al. 2012). this concept is scanty but NSAIDs decreased GFR to a
A notable result is that paracetamol increased the risk of greater extent than placebo or paracetamol in a trial
hypertension in women although bias due to taking para- involving stressed kidneys (low sodium diet, dehydration
cetamol for painful conditions is possible (Curhan et al. and exercise) (Farquhar et al. 1999), and immediately after
2002). In studies on patients treated with antihypertensives, surgery in elderly patients (Koppert et al. 2006). Similarly,
paracetamol has little effect on blood pressure and a lesser ibuprofen depressed renal function to a greater extent than
effect than NSAIDs (Radack et al. 1987; Pavlicevic et al. paracetamol during surgery on sodium-depleted, anaes-
2008; Aljadhey et al. 2012). It is possible that blood thetised dogs (Colletti et al. 1999). Peripheral oedema was
pressure increases in some patient groups and an important also less common in a clinical trial comparing paracetamol
recent finding is that paracetamol increases blood pressure and naproxen (Temple et al. 2006). The renal safety of
by about 3 mmHg in patients with coronary artery disease paracetamol is also indicated by two studies finding no
(Sudano et al. 2010). In clinical practice, it is reasonable to increase in the risk of hospitalisation for heart failure
check for hypertension in patients taking regular paracet- (Merlo et al. 2001) and no worsening of renal function in
amol. Conversely, a temporary decrease in blood pressure patients with grades 45 kidney disease (Evans et al. 2009).
has been noted after intravenous injection of paracetamol Both are conditions in which NSAIDs are expected to
in acutely ill patients (Hersch et al. 2008; de Maat et al. worsen renal function. Conversely, in an epidemiological
2010; den Hertog et al. 2012). study, Fored et al. (2001) reported that paracetamol exac-
Renal prostaglandins and prostacyclin are synthesised by erbated the development of chronic renal failure, although
both COX-1 and COX-2. It is now apparent that NSAIDs bias could not be excluded in this population study.
have little or no effect on renal function in patients with The effect of paracetamol on infarction is of consider-
good renal function but may cause renal impairment in able interest because of its widespread use. The highly
patients with risk factors. These risk factors include already selective COX-2 inhibitor, rofecoxib, was associated with
impaired kidney function (particularly in elderly patients) increased myocardial infarction and its availability was
(Murray et al. 1995; Whelton et al. 2000), dehydration, consequently stopped. There is a small tendency for low
sodium depletion (Colletti et al. 1999; Farquhar et al. 1999) doses of the presently available COX-2 selective inhibitors
heart failure, diabetes, liver disease or taking the combi- and the non-selective NSAIDs to lead to myocardial
nation of a diuretic together with inhibitors of angiotensin infarction while paracetamol appears safe (Latimer et al.
converting enzyme (ACE) or angiotensin receptor (Bouvy 2009).

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Clinical analgesic efficacy of paracetamol (Jordan et al. 2003; Zhang et al. 2005) recommend para-
cetamol as the first-line analgesic of choice for mild to
Acute pain moderate pain in osteoarthritis. Recent recommendations
by the American College of Rheumatology for the treat-
The results of some recent reviews and meta-analyses of ment of osteoarthritis in the hip and knee are not entirely
the analgesic activity of paracetamol and combinations clear cut but still appear to place treatment with full-dose
with other analgesics are summarised in Table 2. Single paracetamol before NSAIDs (Hochberg et al. 2012). If
doses of paracetamol show analgesic activity in a variety of initial treatment with paracetamol is positive, it is recom-
acute pain syndromes; however, a common finding is that mended for continued long-term use for osteoarthritis and
paracetamol is somewhat less effective than NSAIDs. low back pain (Nikles et al. 2005).
Furthermore, paracetamol has, like the NSAIDs and The treatment of the pain of osteoarthritis and other
selective COX-2 inhibitors, better analgesic activity in chronic non-cancer pain is still very difficult and requires
acute post-surgical pain than in the long-term pain of an understanding of the cause of the pain, patient educa-
osteoarthritis (Table 2). However, paracetamol is used tion, realistic expectations, good communication and
extensively and increasingly given intravenously post- regular review and support (Zhang et al. 2005; Milder et al.
operatively as part of multi-modal analgesia regimens. 2010). The programme needs to encompass physical,
psychological and social elements. Weight loss and exer-
Treatment of chronic pain, low back pain, osteoarthritis cise may be very beneficial. Opioid analgesics have
previously been avoided due to misunderstandings about
Chronic pain is a major and common problem with sig- addiction, tolerance and dependence, but Opioid analge-
nificant associated disability and health care burden. sics, with or without paracetamol, are useful alternatives in
Paracetamol provides pain relief in chronic osteoarthritic patients in whom NSAIDs including COX-2 selective
pain although the effect size is small (Table 2). As in the inhibitors (coxibs), are contraindicated, ineffective, and/or
treatment of acute pain, the NSAIDs provide better pain poorly tolerated (Zhang et al. 2005).
relief but the effect size of NSAIDs is still small (Table 2).
Like NSAIDs, paracetamol may decrease the synovitis of Cancer pain
osteoarthritis (Brandt et al. 2006), although, as determined
by serial X-rays, treatment with paracetamol still resulted Paracetamol is widely administered with opioids for the
in a slight deterioration of knee osteoarthritic manifesta- treatment of pain due to cancer. It is listed as an essential
tions in many patients after treatment for 2 years (Williams drug for hospice use (IAHPC 2007). The WHO Pain Relief
et al. 1993). Ladder lists prompt oral administration of drugs in the
There are, however, some important limitations in many following order: non-opioids such as NSAIDs or paracet-
clinical trials. The short duration, often only up to 6 weeks, amol; then combination products for moderate pain
is a clear limitation for a drug which may be used for very containing opioids such as codeine, hydrocodone or oxy-
long periods but continued clinical trial of an inactive codone; then, as necessary, strong opioids such as
placebo is unethical. morphine or transdermal fentanyl, as necessary until the
Despite its lower efficacy than NSAIDs, paracetamol is patient is pain free. This three-step approach is inexpensive
widely recommended as the preferred initial analgesic in and stated to be 7090 % effective (Jadad and Browman
osteoarthritis and low back pain because of its superior 1995). In Europe and Australia paracetamol is routinely
tolerance (Day and Graham 2005; Nikles et al. 2005). In prescribed at step 1 and continued at steps 2 and 3, whereas
terms of cost-benefit, paracetamol is favoured over both the in North America paracetamol is often confined to steps 1
non-selective NSAIDs and the selective COX-2 inhibitors and 2. Patients in severe cancer pain should be treated
even when these drugs are used with proton pump inhibi- immediately with opioids. A variety of other drugs and
tors to reduce their adverse gastrointestinal effects (Latimer treatments, including corticosteroids, anti-depressants,
et al. 2009). This is because the better control of the epidural dosage of analgesics and neurolytical techniques
symptoms by both classes of NSAIDs is outweighed by the may be useful depending upon the cancer and its treatment
cost of treatment of their adverse effects (Latimer et al. (Christo and Mazloomdoost 2008).
2009). The NSAIDs are of course considered if the Despite the recommendations in the WHO Pain Relief
response to paracetamol is inadequate. Ladder, the value of paracetamol in the treatment of cancer
Several international guidelines including those of the pain is still contentious and poorly studied. Stockler et al.
American Geriatric Society (American Geriatrics Society (2004) showed small, though statistically significant ben-
Panel on Chronic Pain in Older Persons 1998) and the efits in pain and wellbeing and concluded that the addition
European League of Associations of Rheumatology of paracetamol is worth considering in all patients with

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The modern pharmacology of paracetamol 205

Table 2 Clinical analgesic activity of paracetamol (P) compared with placebo, NSAIDs and combinations of paracetamol and opioids
Pain Comparison Number of clinical trials, number of patients in first Result References
group, number of patients in comparator group

Osteoarthritis (pain) P vs. placebo 2, 193,198 ESa = 0.21 Zhang

NSAIDs vs. P 11, 824, 814 ES = 0.20 et al.
(2004)
Osteoarthritis (pain) P vs. NSAIDs 3 ES = 0.33 Wegman
et al.
(2004)
Postoperative P 500 mg vs. placebo 6, 290, 271 50 % pain relief Toms et al.
NNTb = 3.5, (2008)
RBc = 1.9
P 600650 mg vs. placebo 19, 954, 932 50 % pain relief
NNT = 4.6,
RB = 2.4
P 9751,000 mg vs. placebo 29, 1,903, 1329 50 % pain relief
NNT = 3.6,
RB = 2.7
Oral surgery Pain P (\1,000 mg) vs. placebo 9, 190, 188 50 % pain relief Weil et al.
relief at 6 h NNT = 6, (2007)
RB = 1.9
P (1,000 mg) vs. placebo 6, 487,690 50 % pain relief
NNT = 3,
RB = 4.2
Post partum pain P (500650 mg) vs. placebo 5, 275, 207 Adequate relief Chou et al.
NNT = 4, (2010)
RB = 1.9
P (1,000 mg) vs. placebo 6, 425, 372 Adequate relief
NNT = 3,
RB = 2.4
Postoperative P (8001,000 mg) ? codeine 4, 153, 151 50 % pain relief Toms et al.
including dental (60 mg) vs. P NNT = 6.1, (2009)
(8001,000 mg) RB = 1.3
Over 46 h P (600650 mg) ? codeine 10, 309, 313 50 % pain relief
(60 mg) vs. P (600650 mg) NNT = 8.2,
RB = 1.3
P (8001,000 mg ? codeine 3, 121, 71 50 % pain relief
(60 mg) vs. placebo NNT = 2.2,
RB = 6.3
P (600650 mg) ? codeine 17, 857, 556 50 % pain relief
(60 mg) vs. placebo NNT = 3.9,
RB = 2.6
Postoperative P (1,000 mg) ? oxycodone 3, 147, 142 50 % pain relief Gaskell
including dental (10 mg) vs. placebo NNT = 1.8, et al.
over 46 h RB = 4.9 (2009)
P (650 mg) ? oxycodone 10, 680, 363 50 % pain relief
(10 mg) vs. placebo NNT = 2.7,
RB = 3.9
Data from meta-analyses and reviews
a
ES = Difference between treated and control groups divided by the standard deviation of the groups; ES of 0.2 is considered small, ES = 0.5
moderate and ES [0.8 large
b
NNT = number of patients needed to be treated with paracetamol or combination to obtain one more patient with pain relief than in the control
(placebo or paracetamol alone)
c
RB = relative benefit = Relative proportions of patients with pain relief in treatment and control (placebo) groups

cancer-related pain. However, others have not found par- are difficult to compare because of selection bias and small
acetamol to offer additional relief of cancer pain over numbers of patients (Tasmacioglu et al. 2009; Zernikow
opioids although non-blinded comparisons of treatments et al. 2006). If paracetamol is being used in combination

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206 G. G. Graham et al.

with an opioid for cancer pain, one clinical approach with intravenous paracetamol often lowers the required opioid
well-controlled pain is to ask the patient to try going dosage in acute pain (Macario and Royal 2011; Tsang et al.
without paracetamol for a couple of days . Those who do 2013) but the adverse effects of opioid treatment may not
feel a difference go back on their regular dose of paracet- be decreased (Maund et al. 2011). Systematic reviews
amol. (Axelsson et al. 2008). examining paracetamol combined with various opioids for
acute postoperative pain have shown increased efficacy
Analgesic effects of combinations with NSAIDs, when combined with codeine and oxycodone (Table 2).
opioids and caffeine in non-cancer pain Combinations of paracetamol and codeine are used
widely but the use of codeine in these preparations has
Paracetamol plus NSAIDs received particular criticism. Codeine is a prodrug, its
metabolism to morphine being responsible for its analgesic
Generally, the combination of NSAIDs and paracetamol efficacy. Ultrafast metabolism to morphine by some
provides greater analgesia than paracetamol alone for the patients may lead to greater relief of pain but an increased
acute pain after orthopaedic, gynaecological and dental sur- likelihood of adverse effects. Conversely, codeine is not
gery (Ong et al. 2010). The contrast between the combination converted to morphine in about 8 % of patients with a
and NSAIDs alone is less clear although 64 % of studies show variant cytochrome P450 2D6, the result being a greatly
that the combination has greater acute analgesic activity than reduced effect.
NSAIDs alone (Ong et al. 2010). More recently, greater Hydrocodone and oxycodone are widely used in com-
activity has been noted for combinations of paracetamol bination with paracetamol, particularly in USA. While
(1,000 mg) and ibuprofen (400 mg) than that produced by these combinations have greater efficacy than paracetamol
combinations of paracetamol (1,000 mg) or ibuprofen alone, the two directly acting opioids present greater abuse
(400 mg) with codeine 30 mg (Daniels et al. 2011). potential than codeine and the high incidence of uninten-
In experimental animals, the combination of paraceta- tional overdose in USA is probably due, at least in part, to
mol and an NSAID produces synergistic effects (Miranda the widespread use of these combinations (see Uninten-
et al. 2006, 2008) or additive actions (Fletcher et al. 1997; tional versus intentional overdose section). A further
Kumar et al. 2010) in tests of anti-nociceptive activity. difficulty is that hydrocodone and oxycodone are also
There have been few studies on the efficacy of the com- subject to metabolic interactions with other drugs. For
bination of paracetamol and an NSAID in the treatment of example, the metabolism of oxycodone is inhibited by
osteoarthritis. However, a recent large-scale trial showed that ketoconazole and induced by rifampicin (Kummer et al.
the combination of paracetamol (3 g daily) and ibuprofen 2011).
(1.2 g daily) generally produced a slightly greater effect than
the same dose of paracetamol alone, but there was no signif- Paracetamol plus caffeine
icant contrast with ibuprofen alone (Doherty et al. 2011).
Although paracetamol does not suppress the inflammation The clinical analgesic activity of single doses of paracet-
of rheumatoid arthritis, combinations with NSAIDs show amol is increased to a small, but statistically significant
greater analgesic and anti-rheumatic activity than the NSAIDs extent, by caffeine (Palmer et al. 2010; Renner et al. 2007).
alone (Seideman 1993; Seideman and Melander 1988). The The mechanism may be the increased rate of absorption of
combination of indomethacin and paracetamol is of particular paracetamol after dosage with caffeine (Renner et al.
note as indomethacin (50 mg daily) and paracetamol (4 g 2007). Conflicting interactions between caffeine have been
daily) has very similar efficacy to a much larger dose of reported in the mouse with caffeine both producing both
indomethacin (150 mg daily) alone (Seideman and Melander lesser analgesia and a greater depression of the synthesis of
1988). Further clinical trials of this type (i.e. a small dose of a nitric oxide (NO) in the spinal cord (Godfrey et al. 2006,
NSAID and a full dose of paracetamol versus a larger dose of Godfrey et al. 2007; see below). One group has reported
an NSAID alone) should be conducted. that PG synthesis is inhibited by caffeine alone (Fiebich
Alternating dosage of paracetamol and ibuprofen has et al. 2000) but confirmation of this observation is required.
been used as an antipyretic treatment in children but is only In recent years, a considerable number of papers have
used if the child does not respond to one drug alone claimed that caffeine potentiates the hepatotoxicity of para-
(Nabulsi 2009). cetamol. However, a critical review of the data has indicated
that there is no significant evidence for such toxicity at ther-
Paracetamol plus opioids apeutic levels, and some studies indicate the opposite effect
(i.e. decreased hepatotoxicity). Furthermore, the studies
The addition of paracetamol to opioids can increase effi- showing greater toxicity have all been conducted at supra-
cacy and provide an opioid-sparing effect. Thus, therapeutic concentrations (Palmer et al. 2010).

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The modern pharmacology of paracetamol 207

Inhibition of the synthesis of PGs and related factors PGs (Fig. 2). Thus, when both COX-1 and COX-2 are
present in cells, COX-2 is the major COX isoenzyme
For many years, the mechanism of action of paracetamol was involved in PG synthesis when the concentrations of ara-
uncertain. The pharmacological and toxicological properties chidonic acid are low (Murakami et al. 2000). At high
of paracetamol are consistent with inhibition of the synthesis concentrations of arachidonic acid, PG synthesis is medi-
of PGs from arachidonic acid, the similar actions being ated largely by COX-1 if both enzymes are present. A
particularly noticeable with the selective COX-2 inhibitors further separation of COX-1 and COX-2 is that COX-1 is
(Table 1). However, in broken cell preparations and partially expressed constitutively and generally produces PGs to
purified COX isoenzymes, only supratherapeutic concentra- modulate physiological processes, whereas COX-2 is
tions of paracetamol inhibit the synthesis of PGs; low inducible and typically produces proinflammatory PGs in
concentrations often increase PG synthesis (Robak et al. response to physiological stresses such as infection and
1978; Bambai and Kulmacz 2000; Swierkosz et al. 2002). inflammation (Lee et al. 2007). However, in some cells in
These findings led to the conclusion that paracetamol does the central nervous system and kidney, COX-2 is present
not produce its therapeutic actions by inhibition of the syn- constitutively (Yaksh et al. 2001).
thesis of PGs and related factors. This conclusion is incorrect. Until recently, all work on the interaction between
Paracetamol inhibits the production of PGs from arachidonic paracetamol considered arachidonic acid as the substrate
acid under specific conditions, namely when the peroxide for the COX isoenzymes. However, a recently discovered
tone of isolated cells is low (see Reasons for the apparent complexity is that selective COX-2 inhibitors and non-
COX-2 selectivity of paracetamol section). selective NSAIDs (through their COX-2 inhibitory activ-
A recent finding is that there are two source enzymes of ity) inhibit the COX-2 selective oxygenation of the
arachidonic acid, the precursors of the intermediate PGs, endocannabinoids, 2-arachidonoylglycerol and ananda-
PGG2 and PGH2 (Nomura et al. 2011). Cytosolic phospho- mide (arachidonoylethanolamide) (Fig. 2; Duggan et al.
lipase A2 hydrolyses phospholipids to liberate arachidonic 2011). The products of the oxidation are PG conjugates that
acid in many tissues but, in brain, liver and lung, the are pharmacologically active (Fig. 2; Woodward et al.
hydrolysis of the endocannabinoid, 2-arachidonoylglycerol 2008). Although not yet documented, it is likely that par-
by monoacylglycerol lipase yields arachidonic acid and may acetamol, as well as COX-2 selective inhibitors, decrease
even be the rate-limiting source of arachidonic acid in these the oxidation of endocannabinoids. The inhibited metabo-
tissues (Fig. 2; Nomura et al. 2011). lism of the endocannabinoids may be responsible in part
for the interactions between endocannabinoids with para-
Bifunctional enzymatic activities of COX-1 and COX-2 cetamol and the NSAIDs (see Endogenous cannabinoids
section). Interestingly, the COX-2 mediated oxidation of
In order to understand the mechanism of action of paraceta- endocannabinoids is also blocked by the R isomers of
mol, it is necessary to outline the enzymology of COX-1 and ibuprofen and related drugs which do not inhibit the oxi-
COX-2. Both enzymes are bifunctional, each enzyme pos- dation of free arachidonic acid by COX-2 (Duggan et al.
sessing two activities: cyclooxygenase and peroxidase 2011).
(Fig. 2). The first function of both enzymes is cyclooxygenase
activity with the oxidation of arachidonic acid to PGG2. It is of Reasons for the apparent COX-2 selectivity
note that PGG2 is a hydroperoxide, with this species being of paracetamol
metabolised subsequently by the peroxidase activities of
COX-1 and COX-2 to PGH2 which, in turn, is converted by Hanel and Lands (1982) reported that paracetamol was a
specific enzymes to prostanoids (Fig. 2). The activity of more potent inhibitor of COX-1 when the levels of per-
COX-1 and COX-2 is dependent on the peroxidase function, oxides are low. The concentrations of paracetamol were
but this can operate independently, i.e. the peroxidase func- still supratherapeutic but indicated a point of separation
tion can oxidise a variety of organic substances in the presence from the non-selective NSAIDs. In contrast, the potency of
of hydrogen peroxide or other peroxides. Paracetamol is one the NSAIDs is not increased by decreasing the concentra-
of the oxidisable substrates of COX-1 (Potter and Hinson tions of lipid peroxides (Hanel and Lands 1982).
1987; Harvison et al. 1988) and it is assumed that the perox- This finding has now been extended to intact cells where
idase function of COX-2 can also oxidise paracetamol. paracetamol-induced blockade of PG synthesis is markedly
reduced by t-butylperoxide (Boutaud et al. 2002; Lucas
Separation of COX-1 and COX-2 pathways et al. 2005). It now evident that therapeutic concentrations
of paracetamol inhibit PG synthesis in intact cells when the
In cells containing both COX-1 and COX-2, there appears concentration of added arachidonic acid is low or the cells
to be a compartmentalisation of pathways that synthesise are stimulated with cytokines, such as interleukin 1b,

123
208 G. G. Graham et al.

Fig. 2 Prostaglandin (PG) synthesis

from precursors, phospholipid (the
principal source in many tissues) and
the endocannabinoid, 2-arachidonoyl
glycerol (present in the brain, liver and
lung; Nomura et al. 2011). Both COX-
1 and COX-2 both have
cyclooxygenase and peroxidase
functions. The endocannabinoids,
2-arachidonoyl glycerol and
anandamide
(arachidonoylethanolamide) are both
substrates for COX-2 forming glycerol
or ethanolamide conjugates with the
intermediate prostaglandins (PGG2-C
and PGH2-C), final prostaglandins
(prostaglandin-Cs), thromboxane A2
(thromboxane A2-C) and prostacyclin
(prostacyclin-C) (Duggan et al. 2011;
Woodward et al. 2008)

which cause the release of low levels of arachidonic acid

(Fig. 3, Boutaud et al. 2002; Graham and Scott 2005). High
concentrations of arachidonic acid markedly reduce the
inhibitory action of paracetamol. It is suggested that low
levels of arachidonic acid lead to low intracellular levels of
PGG2, which is a hydroperoxide and a potent effect of
paracetamol on PG synthesis results.
When concentrations of arachidonic acid are low, the
COX-2 pathway is activated in preference to the COX-1
pathway (Murakami et al. 2000) and, therefore, paraceta-
mol generally appears to be a selective COX-2 inhibitor
in vitro with IC50 values often below 10 lM, well within
the therapeutic plasma concentration range (Boutaud et al.
2002; Graham and Scott 2005; Kis et al. 2004). This
explains why paracetamol has anti-nociceptive or antipy- Fig. 3 Effect of paracetamol on prostacyclin synthesis showing the
retic actions involving COX-2 (Li et al. 2008). In contrast, greater inhibition of paracetamol at lower concentrations of paracet-
paracetamol does not have anti-inflammatory agent in amol (Redrawn from Boutaud et al. 2002)
rheumatoid arthritis or acute gout where the peroxide levels
are likely to be high. By comparison, the classical COX-2 knockout mice (Ayoub et al. 2006). This activity of para-
selective inhibitors, such as celecoxib, are active anti- cetamol could be due, therefore, to inhibition of COX-1
inflammatory agents in the treatment of rheumatoid under conditions of low peroxide tone within the central
arthritis because their effect on COX-2 is not peroxide nervous system. (See Inhibition of COX-3 section).
dependent. Overall, the selectivity of paracetamol appears to be
Paracetamol does not possess significant antiplatelet based on peroxide tone. It should not be termed a selective
activity, a COX-1 dependent action, because of the high COX-2 inhibitor although it often appears to be so.
concentrations of arachidonic acid and peroxides, including A finding of potential clinical interest is an association
PGG2, that are released (Boutaud et al. 2002). This lack of between the levels of a polyunsaturated fatty acid, eico-
significant antiplatelet activity is a major factor which sapentaenoic acid, in plasma and inhibition of the synthesis
makes paracetamol appear COX-2 selective. of PGs and related factors by paracetamol. The plasma
Paracetamol may also inhibit COX-1 dependent PG concentrations of eicosapentaenoic acid are increased by
synthesis if the peroxide tone is low. For example, para- the long-term intake of fish oil, and the synthesis of PGs in
cetamol inhibits acetic acid-induced writhing in mice. The blood of rheumatoid patients is inhibited to a greater extent
writhing (or abdominal constriction) produced by the in patients with higher levels of the fatty acid than in
intraperitoneal injection of acetic acid is still inhibited by patients with lower levels of the same (Caughey et al.
paracetamol in COX-2 knockout mice but is lost in COX-1 2010).

123
The modern pharmacology of paracetamol 209

Molecular mechanisms of action of paracetamol

on COX 1/2 and other haem enzymes

It is now well established that paracetamol inhibits the

production of PGs by acting as a substrate of the peroxi-
dase cycles of COX-1 and COX-2 (Fig. 4) although, as
discussed above, the major effect is often on COX-2
(Boutaud et al. 2002; Graham and Scott 2005; Aronoff
et al. 2006).
The initial species generated on oxidation of paraceta-
mol by COX-1 and COX-2 is the paracetamol phenoxyl
free radical. In vitro, this radical undergoes either rapid
dimerization with another paracetamol radical to yield
dimers, or reduction by glutathione back to paracetamol
with consequent formation of glutathione disulphide
(Fig. 5; Potter and Hinson 1987; Harvison et al. 1988).
Higher polymers may also be produced by additional rad-
ical reactions (Fig. 5). The extent of the formation of these
dimers and higher polymers in vivo and their clinical rel- Fig. 4 Parallel effects of paracetamol on the peroxidase functions of
evance are unknown. the COX isoenzymes (COX-1 and COX-2) and on myeloperoxidase.
COX-1 and COX-2 Oxidation of paracetamol by the peroxidase
In addition to its direct interaction with the peroxidase function to the paracetamol free radical (P) occurs in competition
cycles of COX-1 and COX-2, paracetamol may also with oxidation of a tyrosine residue on the enzyme to a tyrosine
scavenge peroxynitrite, which is an activator of COX phenoxyl radical which is essential for the cyclooxygenase function
enzymes (Schildknecht et al. 2008). of COX-1 and COX-2 (Boutaud et al. 2002; Graham and Scott 2005;
Aronoff et al. 2006). As a result, paracetamol inhibits the cycloox-
Paracetamol is also metabolised by other haem peroxi- ygenase function of COX-1 and COX-2 and hence the production of
dises in addition to the peroxidase function of COX-1 PGG2. Concomitant with the oxidation of paracetamol (Fig. 5), the
COX-2, the most important of which may be the leukocyte- high oxidation state haem species formed on the enzyme as a result of
derived species myeloperoxidase (Fig. 5). This enzyme is the initial reaction with peroxide is reduced. The overall result is that
the oxidation of paracetamol by compound I decreases the cycloox-
released by activated neutrophils, monocytes and some ygenase activities of COX-1 and COX-2. Myeloperoxidase The
tissue macrophages at sites of inflammation in response to oxidation of paracetamol by compound I decreases the formation of
inflammatory stimuli. Although this occurs primarily in hypochlorous acid (HOCl) from chloride. Thus, the chlorination cycle
response to invading pathogens, inappropriate or excessive is reduced while the peroxidase is increased with the resulting
oxidation of paracetamol to free radical species. The free radical
cell stimulation can result in release of myeloperoxidase at species are then mainly converted to the dimer (Koelsch et al. 2010;
the wrong time or place. The subsequent reaction of the Fig. 5)
released peroxidase with H2O2 in the presence of halide, or
pseudohalide, ions results in the generation of the powerful found that insect cells transfected with a splice variant of
oxidants HOCl (hypochlorous acid, from chloride ions), COX-1 from mice were more sensitive to paracetamol than
HOBr (hypobromous acid from bromide ions) and HOSCN cells containing either COX-1 or COX-2. Because of its
(hypothiocyanous acid from thiocyanate ions). These oxi- apparent high sensitivity to paracetamol, the splice variant
dants are cytotoxic/cytostatic to bacteria, but can also of COX-1 was termed COX-3. Despite the greater sensi-
induce severe host tissue damage. The result of the oxi- tivity of insect cells expressing COX-3, it should be noted
dation of paracetamol is decreased synthesis of HOCl, that the IC50 in the insect cells was 450 lM, still well
HOBr and HOSCN. Such inhibition has been detected with above therapeutic plasma concentrations of paracetamol.
both the isolated myeloperoxidase, stimulated neutrophils As COX-3 is a splice variant of COX-1, COX-1 and COX-
(Koelsch et al. 2010) and with the related enzyme, eosin- 3 are formed from a common gene.
ophil peroxidase and also with stimulated neutrophils There has been considerable discussion about this
(Kajer et al. unpublished) (see Inhibition of myeloper- hypothesis. First, there is considerable argument about the
oxidase section). detection of COX-3 in man and its enzymatic activity with
vigorous attack (Kis et al. 2005a, b) and defence of the
Inhibition of COX-3 methodology (Simmons et al. 2005). Three splice variants
of COX-1 are present in man. It is unlikely that the major
A major advance in the mechanism of action of paraceta- splice variant (COX-1b1) is enzymatically active as it is
mol was proposed when Chandrasekharan et al. (2002) truncated protein but at least one of the remaining two

123
210 G. G. Graham et al.

times as much PGE2 as cells containing COX-3. As dis-

cussed above, paracetamol potently inhibits PG synthesis at
low concentrations of arachidonic acid and at low rates of
PG synthesis. Therefore, the more potent effects of para-
cetamol on the cells containing COX-3 may not indicate
any specific activity of paracetamol on this isoenzyme but,
rather, may have resulted from the low rate of PG pro-
duction in COX-3 expressing cells.
A further problem for the acceptance of the COX-3
hypothesis is that only one study has compared the sensi-
tivity to paracetamol of cells expressing COX-1, COX-2
and COX-3. Independent verification of this finding is
required.

Action of paracetamol on diclofenac-induced COX-2

Supratherapeutic concentrations of diclofenac induce the

formation of COX-2 in a macrophage cell line (Ayoub
et al. 2011). In contrast to the loss of activity of paracet-
amol when high levels of peroxides are present in other
cellular incubations (see Reasons for the apparent COX-2
selectivity of paracetamol section), a therapeutic con-
centration of paracetamol (10 lM) inhibits PG production
by the macrophage cell line even when t-butyl hydroper-
oxide is added (Ayoub et al. 2011). However, an unusual
result in these experiments was the absence of a concen-
trationresponse relationship with similar levels of
inhibition at 10, 100 and 1,000 lM paracetamol. A prob-
lem in the interpretation of these results is that very high
Fig. 5 Metabolism of paracetamol by the peroxidase function of concentrations of diclofenac were removed from the cells
COX isoenzymes and by myeloperoxidase. Three of the five by a single wash of the medium. Residual diclofenac may
canonical forms of the free radical of paracetamol (P) are shown. have reduced the cyclooxygenase activity sufficiently to
The radicalradical interaction yields at least two dimers of paracet-
amol, the major being 3,30 diparacetamol. Two-electron oxidation by
allow paracetamol to inhibit PG production. Nevertheless,
cytochrome P450 produces the hepatotoxic metabolite N-acetyl-p- the results of Ayoub et al. (2011) should be studied further.
quinone imine (NAPQI) which is deactivated by reaction with The first is a more general examination of the induction of
glutathione (GSH). Some NAPQI is also formed by the peroxidase COX-2 by NSAIDs. The effect of paracetamol on this
function of the COX isoenzymes
induced COX-2 has potential clinical significance and
should also be examined.
minor splice variants (COX-1b2) does have enzymatic
activity (Qin et al. 2005). However, paracetamol shows no Time-dependent pathways of PG synthesis
preferential inhibition of this isoform in a cell free system and paracetamol action
and, as expected, the IC50 is very high ([2 mmol/L) (see
Reasons for the apparent COX-2 selectivity of paraceta- As noted above, COX-1 is constitutive while COX-2 is
mol section). often up-regulated by infection and inflammation. In the
A second problem has not been considered widely but spinal cord, however, COX-2 is constitutive although still
may very well invalidate the COX-3 hypothesis as the up-regulated by peripheral inflammation (Beiche et al.
major molecular site of action of paracetamol. This comes 1996). COX-1 is actually down-regulated in oral mucosa
from consideration of the kinetic details of the original after oral surgery, and it has been suggested that paracet-
finding (Graham and Scott 2005). In the absence of para- amol may inhibit COX-1 in the early stages of
cetamol, insect cells containing COX-1 produced about inflammation but with continuing inflammation, COX-2 is
five times as much PGE2 as cells containing COX-3 while induced (Khan et al. 2007; Lee et al. 2007) and is then
cells containing the COX-2 produced approximately 25 subject to inhibition by paracetamol.

123
The modern pharmacology of paracetamol 211

Genetically mediated expression of COX-1 and COX-2 with the analgesic activity of paracetamol, the antipyretic
activity appears to be due to COX-2 blockade, as no effect
While the expression of COX-1 typically decreases after of paracetamol is seen in COX-2 knockout mice. Further-
oral surgery and COX-2 is generally induced, there are more, lipopolysaccharide and interleukin-1b do not cause
wide inter-subject variations in the altered expression of fever in COX-2 knockout mice but they do produce fever
the two enzymes. Such inter-patient variation has been in COX-1 knockout mice. As COX-3 is a spice variant of
associated with changes in the analgesic actions of ibu- COX-1, the antipyretic action of paracetamol cannot be
profen and rofecoxib (Lee et al. 2006) and could also due to inhibition of COX-3 (Li et al. 2008). Also, selective
contribute to the variable response to paracetamol. COX-2 inhibitors are antipyretic in man.
In summary, paracetamol often appears to show COX-2
selectivity but may inhibit COX-1 dependent PG synthesis Anti-inflammatory effect
in the presence of low levels of arachidonic acid. However,
paracetamol is not entirely analogous to the selective COX- The major pharmacological difference between paraceta-
2 inhibitors because paracetamol also inhibits myeloper- mol and COX-2 inhibitors is the widely cited lack of effect
oxidase and other peroxidases (see Inhibition of of paracetamol on the inflammation of rheumatoid arthritis.
myeloperoxidase section). Paracetamol is not acutely anti-inflammatory in rheumatoid
arthritis (Boardman and Hart 1967; Ring et al. 1974;
Relationship of COX-2 inhibition to the therapeutic Table 3). Correspondingly, paracetamol does not suppress
and adverse actions of paracetamol PGE2 concentrations in synovial fluid in rheumatoid
patients whereas the NSAIDs do (Seppala et al. 1985).
As outlined above, the pharmacological and toxicological However, paracetamol does suppress inflammation under
effects of paracetamol are similar to those of the NSAIDs, conditions of lesser inflammation: after dental extraction
particularly the COX-2 selective inhibitors, such as cele- (Bjrnsson et al. 2003), possibly in osteoarthritis and in a
coxib and etoricoxib (Table 1). The ability of paracetamol variety of inflammatory tests in experimental animals
to inhibit PG synthesis in cellular systems and in vivo (Table 3). The suppression of low-grade inflammation by
provides good evidence for its primary action being inhi- paracetamol may be due to low levels of arachidonic acid
bition of the synthesis of PGs and related factors. and/or peroxides; conditions where paracetamol is a potent
inhibitor of PG synthesis (Fig. 3). Anti-inflammatory
Analgesia activity may be possible in rheumatoid arthritis with high
doses but toxicity prevents this use.
There is considerable evidence that the analgesic effect of
paracetamol is related to its inhibition of the synthesis of Safety in the gastrointestinal tract
PGs and related factors. First, PGs potentiate the pain
produced by pain mediators such as bradykinin and para- PGs are cytoprotective in the stomach and are synthesised
cetamol inhibits bradykinin-induced pain in animal models by the combination of COX-1 and COX-2 activities and
(Botha et al. 1969). Second, paracetamol not only inhibits inhibition of both pathways is established as the major
of PG synthesis but also decreases PGE2 concentrations of cause of the gastrointestinal toxicity of the non-selective
in vivo simultaneously with it anti-nociceptive effects in NSAIDs. A great attribute of paracetamol is that it has no
experimental animals (Muth-Selbach et al. 1999; Lee et al. significant toxicity on the upper gastrointestinal tract
2007; Crawley et al. 2008). Inhibition of the synthesis of (Graham et al. 2001). In this regard, paracetamol resem-
PGs and related factors triggers changes in several neural bles the selective COX-2 inhibitors (Table 1) and is the
systems (see Linkages to other neuronal systems sec- major reason for the superior tolerance of paracetamol
tion) but, taken together, the data suggest that the primary over the non-selective NSAIDs. As it has excellent gas-
effect of paracetamol is its inhibition of the synthesis of trointestinal tolerance, paracetamol is a suitable mild
PGs and related factors with changes in other neural analgesic for patients with a history of peptic ulcer
pathways being secondary to this. (Nielsen et al. 2006). However, a loss [10 g/L haemo-
globin has been reported in 7 % of patients treated with
Antipyretic effect 3 g paracetamol daily (Doherty et al. 2011). This result
may indicate gastrointestinal blood loss in some patients
Pyrogens increase the concentrations of PGE2 in cerebro- even though, on average, there is no significant decline in
spinal fluid and mediates pyresis (Ivanov and Romanovsky haemoglobin levels. Possibly, paracetamol produces some
2004). This increase is blocked by paracetamol (Feldberg damage to the small intestine, as has been noted with
et al. 1973; Dey et al. 1974; Li et al. 2008). As is the case celecoxib and etoricoxib.

123
212 G. G. Graham et al.

Table 3 The variable anti-inflammatory effects of paracetamol

Method Result in presence of paracetamol Comments References

No anti-inflammatory effect
Circumference of fingers No effect vs placebo High dose of paracetamol (6 g daily) Boardman
and thumbs (jewellers and Hart
rings) (1967)
Thermography No effect Moderate dose of paracetamol (3 g daily) Ring et al.
(1974)
Positive anti-inflammatory effect
Carrageenan induced Both reduced by paracetamol Vinegar et al.
pleurisy and adjuvant (1976)
arthritis in rats
Carrageenan inflammation Inflammation reduced Glenn et al.
of rat paws (1977)
Adjuvant arthritis in rats Inflammation and bone degeneration Synergistic activity of paracetamol and tolmetin Wong and
reduced Gardocki
(1983)
Carrageenan-induced Inflammation reduced. Inflammation enhanced by arachidonic acid but Lewis et al.
inflammation of rat paws enhanced inflammation was not inhibited by (1975)
paracetamol
Carrageenan-induced Inflammation reduced Concurrently decreased Fos in dorsal horn Honore et al.
inflammation of rat paws (1995)
Local swelling after oral Swelling reduced by 30 % compared to Lkken and
surgery in man placebo. Skjelbred
(1980)
Local swelling after oral Swelling reduced by paracetamol Similar effect to ibuprofen (2,400 mg daily) Bjrnsson
surgery in man (total 4 g daily) et al.
(2003)
Swelling after orthopaedic Swelling reduced to very similar extent by Mburu et al.
surgery in dogs. paracetamol (33 %) compared to aspirin (1988)
(24 %)
Synovial volume in Volume decreased to similar extent as No placebo Brandt et al.
osteoarthritis ibuprofen (2006)

Bronchoconstriction and asthma possibility of an epigenetic phenomenon, i.e. an heritable

change without changes in the DNA sequences.
Paracetamol precipitates asthma in about 7 % of aspirin- There are suggestions that the association between
sensitive asthmatics although the syndrome is shorter and paracetamol use in children and the development of asthma
more easily treated than if caused by aspirin (Jenkins et al. is confounded. Confounders may include the use of para-
2004; Szczeklik et al. 2002; Table 1). The selective COX-2 cetamol in children with asthma leading to a greater risk of
inhibitors do not induce asthma in aspirin-sensitive asth- respiratory and ear infections and, consequently, more
matics (Stevenson 2004) whereas the non-selective common treatment with paracetamol (Chang et al. 2011;
NSAIDs all cross-react with aspirin. The weak effect of Lowe et al. 2012). Most reviewers of this topic have stated
paracetamol is consistent with its predominant selectivity that randomised prospective trials are required to determine
for COX-2. if there is a causal relationship between paracetamol and
Despite the weak induction of acute asthma by para- asthma. This would, however, be a very difficult task.
cetamol, several epidemiological studies indicate that Nevertheless, it is rational to check respiratory function in
paracetamol increases risk of asthma in children and adults children taking regular paracetamol.
(Etminan et al. 2009; Beasley et al. 2011). Exposure of The mechanism of any paracetamol-induced risk of
children to asthma could occur by long-term changes in the asthma is unclear, but recent work in mice indicates that
expression of inflammatory mediators, but this mechanism paracetamol produces localised inflammation and increases
does not explain the association between asthma and pre- the expression of myeloperoxidase in the trachea (Nassini
natal exposure to paracetamol (Shaheen et al. 2010; Eyers et al. 2010). These effects appear to be due to activation of
et al. 2011). The influence of prenatal dosage raises the transient receptor potential ankyrin-1 (TRPA1) cation

123
The modern pharmacology of paracetamol 213

channel by NAPQI, the hepatotoxic metabolite of para- many studies have now shown that NSAIDs, like paraceta-
cetamol, as these effects are produced by the intratracheal mol, have both central and peripheral actions (see below).
application of NAPQI and are not produced in TRPA1
knockout mice. As outlined in this review, inhibition of Central and peripheral sites of action of paracetamol
myeloperoxidase by paracetamol is considered as an anti-
inflammatory effect but this effect on the trachea appears to A significant site of action of paracetamol is the central nervous
be an inflammatory response to the drug. This up-regula- system where systemic administration inhibits the rise in
tion of myeloperoxidase requires careful examination in central PGE2 produced by peripheral application of pyrogens
human studies. or painful stimuli (Feldberg et al. 1973; Muth-Selbach et al.
1999). Paracetamol also decreases PGE2 production in the
Anti-platelet effect central nervous system after the intraperitoneal injection of
acetic acid in mice (writhing test) (Ayoub et al. 2006).
Paracetamol has little anti-platelet activity, another point of A further indication of the central effect of paracetamol is
similarity with the selective COX-2 inhibitors. Paracetamol the effect of injection of small doses of paracetamol directly
does, however, inhibit the production of thromboxane A2 into the central nervous system intrathecally (i.t.) or intra-
and platelet aggregation at peak concentrations when used cerebroventricularly (i.c.v.) (Table 4) (Malmberg and Yaksh
at high doses, particularly those achieved after intravenous (1992a); Alloui et al. (1996)). Paracetamol blocks the
dosage (Lages and Weiss 1989; Niemi et al. 2000; Mun- nociceptive behaviour produced by the i.t. administration of
sterhjelm et al. 2003; Munsterhjelm et al. 2005); this central neurotransmitters, including substance P (Crawley
activity is, however, lost rapidly because of the short half- et al. 2008; Choi et al. 2001; Bjorkman et al. 1994), gluta-
life of paracetamol. mate (Choi et al. 2001) and the 5-HT-3 receptor antagonist,
An advantage of paracetamol over the non-selective tropisetron (Pelissier et al. 1996). All these observations
NSAIDs, such as ibuprofen and naproxen, is that paracet- indicate a central site of action of paracetamol.
amol does not interfere with the antiplatelet activity of There is also evidence for peripheral effects of paraceta-
aspirin (Catella-Lawson et al. 2001; Gladding et al. 2008). mol. For example, the local (intraplantar) injection of very
Thus, paracetamol can be used with low-dose aspirin which small doses of paracetamol decreases neuropathic pain in
is now widely used for the prevention of cardiac re- rats (Dani et al. 2007) Low doses (approximately 30 mg)
infarction. The selective COX-2 inhibitors also do not block applied as a gel to the tooth socket also decrease pain after
the antiplatelet activity of aspirin. The lack of effect of both molar extraction, an indicator of a direct peripheral effect
the paracetamol and selective COX-2 inhibitors is a factor (Moore et al. 1992). Correspondingly, the systemic admin-
adding to the apparent COX-2 selectivity of paracetamol. istration of paracetamol decreases pain and PGE2
concentrations in surgical sites following removal of molar
teeth (Lee et al. 2007). The same effect on PGE2 occurs with
Sites of action rofecoxib. In vitro, paracetamol inhibits PG synthesis by a
variety of peripheral cells as well as cells derived from the
Paracetamol is widely stated to have only a central site of central nervous system (Graham and Scott 2005).
action as opposed to peripheral sites of action of the Overall, both central and peripheral actions of paracet-
NSAIDs and selective COX-2 inhibitors. This is incorrect. amol are indicated.
The theory of a preferential site of action of paracetamol in
the central nervous system arose primarily from the widely Central and peripheral sites of action of NSAIDs
cited work of Flower and Vane (1972) who found that and selective COX-2 inhibitors
paracetamol produced more potent inhibition of the syn-
thesis of PGs and related factors in the microsomal fraction The inhibition of the synthesis of PGs and related factors in
of rabbit brain than dog spleen. With the aid of hindsight, peripheral systems has been widely observed with NSAIDs
one can see that the experimental method was faulty in that and selective COX-2 inhibitors. However, i.t. injections of
the activity of the paracetamol was measured when the classical NSAIDs and selective COX-2 inhibitors also have
cofactor was hydroquinone, an oxidisable cofactor. The anti-nociceptive activity (Bannwarth et al. 1995; Nishiy-
utility of using one phenol, hydroquinone, to measure the ama 2006). Furthermore, as discussed below, non-selective
activity of another phenol, paracetamol, is unsound. NSAIDs block the effects of i.t. injections of glutamate or
The peripheral site of action of NSAIDs came originally substance P (Table 4), again indicating a central site of
from the studies of Lim et al. (1964). By cross-perfusion action of NSAIDs.
experiments in dogs, Lim et al. (1964) concluded that the In summary, paracetamol, NSAIDs and selective COX-2
analgesic activity of aspirin was purely peripheral. However, inhibitors all have both central and peripheral effects.

123
214 G. G. Graham et al.

Table 4 Interactions of paracetamol, NSAIDs and selective COX-2 inhibitors with other neural systems and neurotransmitters
Interacting neurotransmitter Principal results References
and system

Serotonin Antinociceptive effect of paracetamol is inhibited by:

(5-hydroxytryptamine,
5-HT)
5-HT-3 antagonists in mice Alloui et al. (1996, 2002)
5-HT-2 antagonist in rats Ruggieri et al. (2008),
Pelissier et al. (1996)
5-HT-3 antagonist in humans Pickering et al. (2006,
2008), Bandschapp
et al. (2011)
Destruction of descending 5-HT pathways by 5,7-dihydroxytryptamine in mice Mallet et al. (2008)
Depletion of brain content of 5-HT by p-chlorophenylalanine Miranda et al. 2003
Antinociceptive effect of aspirin is inhibited by 5-HT blockers and by Sandrini et al. (2002);
p-chlorophenylalanine Vitale et al. (1998)
Anti-nociceptive effect of several NSAIDs and selective COX-2 inhibitors inhibited by Miranda et al. (2003)
blockade of 5-HT synthesis by depletion of brain 5-HT by p-chlorophenylalanine
Opioid Anti-nociceptive effect of paracetamol inhibited by l and j opioid receptor antagonists Pini et al. (1997)
in rats Sandrini et al. (2001)
Ruggieri et al. (2008)
Analgesic effect of COX-2 inhibitors blocked by naltrexone Franca et al. (2006)
Analgesic effect of ketorolac in mice is reduced by naloxone Domer (1990)
Cannabinoid Anti-nociceptive effect of paracetamol blocked:
By CRB1 antagonist Ottani et al. (2006)
In CRB1 knockout mice Mallet et al. (2008)
By chronic exposure of mice to tetrahydrocannabinol Anikwue et al. (2002)
Cannabinoid receptor antagonists block the effects of local (intraplantar) injections of Dani et al. (2007)
paracetamol
Anti-nociceptive effect of an i.c.v. cannabinoid agonist is enhanced by i.c.v. Ahn et al. (2007)
paracetamol
Subeffective dose of paracetamol potentiates the anxiolytic effect of an Umathe et al. (2009)
endocannabinoid
Anti-nociceptive effect of several NSAIDs and celecoxib is blocked by chronic Anikwue et al. (2002)
exposure of mice to tetrahydrocannabinol
Anti-inflammatory effect of indomethacin is blocked by CB2 antagonist in mice Holt et al. (2005)
Anti-nociceptive effect of indomethacin is blocked by CB1 antagonist and in CB1 Guhring et al. (2002),
knockout mice Ates et al. (2003)
Noradrenaline Synergistic interaction with clonidine, an a2 agonist which decreases sympathetic Miranda and Pinardi
outflow from central nervous system (2004)
Synergistic interaction with phentolamine in abdominal constriction test in mice. Both Raffa et al. (2001)
drugs dosed i.t.
Acetylcholine Synergistic interaction with acetylcholinesterase inhibitor (neostigmine) in abdominal Miranda et al. (2002)
constriction test in mice
Glutamate, N- Paracetamol inhibits nociceptive behaviour of rats and mice after i.t. NMDA, glutamate Bjorkman et al. (1994)
methylaspartate and or substance P Choi et al. (2001)
substance P
NSAIDs decrease the excessive sensitivity to pain (hyperalgesia) induced by the Malmberg and Yaksh
activation of spinal glutamate and substance P receptors (1992b)
Nitric oxide (NO) Anti-nociceptive action of paracetamol after i.t. NMDA or substance P is reversed by Bjorkman et al. (1994)
precursor of NO, L-arginine
Inhibitors of NO synthase potentiate anti-nociceptive effect of paracetamol. Bujalska (2004)
Paracetamol may inhibit NO synthesis in the spinal cord. Godfrey et al. (2007)
Anti-nociceptive activity of i.t. indomethacin not shown in mice lacking inducible NO Guhring et al. (2000)
synthase (iNOS knockout mice)

123
The modern pharmacology of paracetamol 215

Linkages to other neuronal systems paracetamol, there are several studies indicating that the
analgesic activity of the NSAIDs and COX-2 inhibitors
The anti-nociceptive actions of paracetamol are substan- appears to be dependent on intact central 5-HT systems
tially linked to other neuronal systems. There are two (Table 4).
general types of linkage to other systems. One type of
linkage is essential to the activity of paracetamol and may Opioids
even be activated by paracetamol. The analgesic activity of
paracetamol is blocked by inhibitors of these systems. Most The analgesic activity of paracetamol is reversed in
notably, the anti-nociceptive effects of paracetamol are experimental animals by opioid antagonists (Table 4),
decreased by inhibitors of serotonin (5-hydroxytryptamine, indicating that this action of paracetamol involves endog-
5-HT), endogenous opioids, endogenous cannabinoids and enous opioids. An indication of linkage between
possibly acetylcholine (Table 4). paracetamol and opioids and the further association with
A second general mechanism of interaction is provided serotonergic systems is the observation that naloxone
by neurotransmitters or factors whose effects are inhibited inhibits the paracetamol-induced increase in 5-HT and
by paracetamol. Blockers of systems involving these fac- decreased 5-HT2 receptors in the cerebral cortex (Pini et al.
tors are expected to potentiate the anti-nociceptive action 1997). An interaction of COX-2 inhibitors with opioid
of paracetamol. Examples include the nociceptive effects systems is also evident as the opioid antagonist, naltrexone,
of substance P, glutamate, fos and, possibly, noradrenaline can also block the analgesic effects of COX-2 inhibitors in
(Table 4). rats (Table 4).
Both classes of anti-nociceptive effects can be related to Possible effects of paracetamol on opioid systems in
inhibition of the synthesis of PGs and related factors by human have not been examined well. However, the anal-
paracetamol as they are generally also shown by non- gesic effect of paracetamol has been reported to correlate
selective NSAIDs and selective COX-2 inhibitors with a decrease in the plasma concentrations of beta-
(Table 4). endorphin in osteoarthritis (Shen et al. 2006).
A significant problem in relating the therapeutic effect
of paracetamol to neuronal systems is that experimental Endogenous cannabinoids
data are often inconsistent, with only some investigators
observing positive results. Bannwarth et al. (1995) sug- Endocannabinoids are involved in spinal pathways of pain
gested that these inconsistencies were related to the bell- and the involvement of endocannabinoids in the analgesic
shaped doseresponse relationships seen with the hyper- action of paracetamol is indicated by its reduced activity
algesic effects of PGs (Uda et al. 1990). However, the full under conditions of decreased activity of the endocannab-
reasons for the inconsistencies remain to be resolved. inoid system (Table 4). As with interactions with 5-HT and
endogenous opioids, an interaction with endocannabinoids
5-Hydroxytryptamine (5-HT, serotonin) is also shown by non-selective NSAIDs (Guhring et al.
2002; Ates et al. 2003; Fowler, 2004). Also, like a non-
The maintenance or activation of some serotonergic sys- selective NSAID and a selective COX-2 inhibitor, para-
tems is essential for the analgesic effect of paracetamol. cetamol enhances the anti-nociceptive activity of a
This has been demonstrated convincingly in experimental cannabinoid agonist in rats (Ahn et al. 2007).
animals (Table 4). The clinical involvement of 5-HT sys- Overall, it appears that maintenance of the endocan-
tems is indicated by the recent findings that the analgesic nabinoid system contributes to the analgesic activity of
activity of paracetamol is decreased by the 5-HT3 receptor paracetamol. Several mechanisms may contribute to the
antagonists, tropisetron and granisetron, in human volun- involvement of endocannabinoids in the analgesic actions
teers although ondansetron and tropisetron did not decrease of paracetamol and NSAIDs. First, COX inhibitors block
the analgesic effect of paracetamol after surgery in man the conversion of arachidonic acids to PGs and, conse-
(Jokela et al. 2010; Pickering et al. 2011). quently, the central metabolism of arachidonic acid may be
In rats, the analgesia produced by paracetamol is diverted to endocannabinoids (Fowler 2004). A second
blocked by a 5-HT1A receptor antagonist, but the analgesic mechanism may be direct inhibition of the COX-2 medi-
activity of the NSAID, diclofenac, is not (Bonnefont et al. ated oxygenation of endocannabinoids (see Inhibition of
2007). This result provides an argument for the analgesic the synthesis of PGs and related factors section). Colo-
activity of paracetamol being unrelated to inhibition of the calisation of serotonergic and cannabinoid receptors within
COX enzymes. However, although the interaction of 5-HT the central nervous system may also lead to crosstalk
mechanisms has been studied to a much lesser degree with between the two systems (Hermann et al. 2002). As has
NSAIDs or selective COX-2 inhibitors than with been discussed above, the involvement of serotonergic

123
216 G. G. Graham et al.

pathways in the analgesic activity of paracetamol could Nitric oxide (NO)

lead, in turn, to an interaction of paracetamol with can-
nabinoid systems. Although the data are sometimes conflicting, NO is gen-
erally considered to potentiate pain responses in the central
Cholinergic systems nervous system (Hamza and Dionne 2009). It appears that
the analgesic effect of paracetamol may involve NO but
The anti-nociceptive activity of paracetamol shows syner- the mechanism is unclear (Table 4).
gistic activity with the well-known anticholinesterase,
neostigmine, when the drugs are administered intraperito- Fos
neally or intrathecally (Table 4). Similar interactions are
known with several non-selective NSAIDs and selective Fos is an early response protein which is up-regulated in
COX-2 inhibitors (Miranda et al. 2002). These results many cells following noxious stimuli and, in the spinal
indicate that the maintenance of central cholinergic sys- cord, is an indicator of neurone activation. Simultaneously
tems may be important in the actions of paracetamol. with blockade of the inflammatory effect of intraplantar
carrageenan, paracetamol decreases the number of neuro-
Central noradrenergic systems nes expressing Fos in the dorsal horn after the intraplantar
injection in the rat (Honore et al. 1995; Abbadie and
The anti-nociceptive activity of paracetamol is increased Besson 1994). Again, this effect is also shown by the non-
by clonidine, an a2 agonist which decreases sympathetic selective NSAIDs, aspirin (Honore et al. 1995; Abbadie
tone (Table 4). As with other anti-nociceptive tests, the and Besson 1994), diclofenac (Buritova et al. 1995) and
interaction with clonidine is produced with non-selective both intrathecal ketorolac and celecoxib (Lee and Seo
NSAIDs and selective COX-2 inhibitors. (Table 4; Mir- 2008).
anda and Pinardi 2004) There are conflicting results with Spinal fos is down-regulated by the intrathecal injection
the non-selective a blocker, phentolamine, which has been of a NSAID and a selective COX-2 inhibitor indicating a
associated with both potentiation (Raffa et al. 2001) and local effect of these drugs on fos expression.
antagonism (Hu et al. 1994) of paracetamol-induced anti-
nociception. The contrast may be related to the very dif-
ferent tests: abdominal irritation (Raffa et al. 2001) and Action through the formation of N-
stimulation of splanchnic nerve (Hu et al. 1994). arachidonylphenolamine (AM404)

Glutamate and substance P A novel mechanism of action was indicated in 2005 when a
bioactive paracetamol metabolite, N-arachidonylphenol-
Paracetamol blocks the apparent nociceptive behaviour pro- amine, was found in mouse brain after dosage with
duced by the i.t. injection of both the excitatory amino acid, paracetamol. (Hogestatt et al. 2005) AM404 is a conjugate
glutamate, and substance P (Table 4). Aspirin similarly of arachidonic acid and p-aminophenol (p-hydroxyaniline)
inhibits the glutamate-induced nociceptive behaviour but (Fig. 6). This metabolite inhibits endocannabinoid uptake
appears inactive against substance P-induced nociception (Beltramo et al. 2000) and has been considered an impor-
(Choi et al. 2001) while several NSAIDs inhibit the hyper- tant mediator in the analgesic properties of paracetamol
algesia produced by glutamate and substance P (Table 4). but, as far as we are aware, only one research paper

Fig. 6 Suggested metabolism

of paracetamol to AM404
(arachidonyl conjugate of
p-aminophenol) and the futile
cycling of paracetamol by
consecutive deacetylation and
reacetylation. The main
function of FAAH (fatty acid
amide hydrolase) is to hydrolyse
endocannabinoids, anandamide,
but it may act in the reverse
direction to synthesise AM404

123
The modern pharmacology of paracetamol 217

Table 5 Experimental results in favour and against the hypothesis AM404 that is synthesised from paracetamol and mediates the pharmaco-
logical effects of paracetamol
In favour Against

AM404 is present in mouse brain after dosage with paracetamol. The Deuterium labelled AM404 and p-aminophenol were detected in rat
presumed intermediate p-aminophenol is also present (Fig. 6; brain after the administration of deuterium-labelled paracetamol but
Hogestatt et al. 2005) no unlabelled AM404 or p-aminophenol was detected (see text)
(Hogestatt et al. 2005)
Inhibitors of fatty acid amide hydrolase (FAAH; Fig. 6) block the Inhibitor of FAAH does not block the action of paracetamol in the
formation of AM404 (Hogestatt et al. 2005) and analgesic effect writhing test in mice (Soukupova et al. 2010)
(Mallet et al. 2008) of paracetamol
AM404 has anti-nociceptive activity. AM404 inhibits allodynia in rats, Ketanserin blocks the anti-nociceptive action of paracetamol but not
an activity which is blocked by CB1 antagonist (Ruggieri et al. 2008) that of AM404 (Mitchell et al. 2007)
AM404 ([0.1 lM) inhibits the production of PGs which are mediators Low concentrations of AM404 (*0.01 lM) present in rat brain do not
of pain, fever and inflammation (Hogestatt et al. 2005) appear sufficient to block COX-1 or COX-2 (Bertolini et al. 2006)

documents the synthesis of AM404 and the presence of paracetamol-induced analgesia. Unfortunately, the effects
p-aminophenol in vivo after the administration of para- of NSAIDs and selective COX-2 inhibitors on these path-
cetamol. Confirmation of the results of Hogestatt et al. ways have not been studied and it is not known if the
(2005) is required for support of the AM404 hypothesis. upregulation seen with paracetamol is definitely related to
Several findings question the AM404 hypothesis inhibition of COX-1 or COX-2.
(Table 5). Not only are there problems in correlating the The upregulation of COX-2 is common in several
actions of paracetamol and AM404 but also, as noted above, inflammatory states. Paracetamol and rofecoxib further
the actions of paracetamol on peroxidases and the peroxidase upregulate COX-2 gene expression after oral surgery in
function of COX isoenzymes are consistent with a direct man although their influence is small compared with the
inhibitory action of paracetamol on PG production without change produced by the surgically induced inflammation
metabolism to AM404. It seems likely that paracetamol, as and pain themselves (Lee et al. 2007). The expression of
well as NSAIDs and selective COX-2 inhibitors, interact COX-2 in this peripheral system is very variable and may
with endocannabinoid systems though their inhibition of the be partly responsible for the considerable inter-patient
synthesis of PGs and related factors. These interactions are variation in the response to paracetamol and other selective
consistent with a COX-2 inhibitory role of paracetamol and COX-2 inhibitors (Lee et al. 2006). Interleukin 1b up-
do not require metabolism to AM404. regulates the expression of COX-2 and, not surprisingly,
Metabolism of paracetamol through the intermediate, there is up-regulation of the gene for interleukin 1b in this
p-aminophenol, is however, supported by the futile model, but neither paracetamol nor rofecoxib altered its
deacetylation of paracetamol (Fig. 6; Nicholls et al. 1997). expression (Lee et al. 2007).
In this metabolic pathway, the acetyl group of 12 % of an Another feature of the oral surgical model is the
oral dose is removed to form p-aminophenol which is then approximately 20-fold increase in the gene expression for
reacetylated to form paracetamol again. Further experi- type IIA secretory phospholipase A2 (Lee et al. 2006). This
mental evidence of this pathway of futile deacetylation is, enzyme hydrolyses phospholipids to release arachidonic
however, required. acid but also has an enzyme independent signalling function
to up-regulate cytosolic phospholipase A2 (cPLA2) and
COX-2 leading to greater PG synthesis (Bryant et al. 2011).
Genetic interactions and future possibilities The influence of paracetamol on the expression of this gene
has not been recorded although ibuprofen and rofecoxib had
There are few reports of genetic interactions of paraceta- no significant effect on its expression (Lee et al. 2006). Type
mol or NSAIDs. With the production of inflammation and IIA sPLA2 is also present in other sites associated with pain
pain in the formalin test in rats, paracetamol changes the and inflammation (e.g. rheumatoid synovium). Inhibitors of
expression of several spinal systems including increased this enzyme have been developed and tested in rheumatoid
expression of receptor for insulin-like growth factor-1. The arthritis, but although there was an initial effect, efficacy was
insulin-like growth factor appears significant in paraceta- not maintained over 12 months (Bradley et al. 2005).
mol action because its analgesic activity is blocked by an Interestingly, an inhibitor of sPLA2 shows centrally medi-
antagonist of the receptor for the growth factor (Bonnefont ated analgesic actions (Svensson et al. 2005).
et al. 2007). Up-regulation of phosphorylated (i.e. acti- It would be of interest to test the actions of inhibitors of
vated) ERK1/2 also occurs and has been also linked to sPLA2, cPLA2 or monoacylglycerol lipase (see Inhibition

123
218 G. G. Graham et al.

of the synthesis of PGs and related factors section) with formation of hypochlorous acid (HOCl) from hydrogen
paracetamol to determine if decreased release of arachi- peroxide and chloride ions and the inappropriate produc-
donic acid potentiates the analgesic actions or even the tion of HOCl is widely considered to be involved in the
anti-inflammatory effect of paracetamol. Sequential oxidative damage to tissues that are subject to acute or
blockade could lead to marked reduction in PG synthesis chronic inflammation (Davies 2011; Davies et al. 2008; van
although gastrointestinal damage could result from the der Veen et al. 2009).
generalised inhibition of the synthesis of PGs and related Paracetamol decreases the production of hypochlorite by
factors. myeloperoxidase with an IC50 value of about 80 lmol/L
In the trachea of mice, paracetamol increases the (Graham et al. 1999; Koelsch et al. 2010). Paracetamol also
expression of myeloperoxidase, an effect which, if also inhibits the activity of myeloperoxidase in intact neutro-
produced in humans, may increase the bronchoconstriction phils. As is the case with the peroxidase function of COX-
and asthma (see Inhibition of myeloperoxidase and 1, inhibition of myeloperoxidase results from the oxidation
bronchoconstriction and asthma sections). of paracetamol to fluorescent polymers, mainly diparace-
tamol (Figs. 4, 5). Interestingly, the IC50 of the major
dimer is approximately 38 lM and thus is approximately
Cardiovascular effects of paracetamol: adverse twice as potent as paracetamol itself (Kajer et al.
or beneficial? unpublished).
As well as inhibiting the production of HOCl, para-
Vascular effects cetamol may inhibit the generation of other powerful
oxidants by myeloperoxidase, including hypobromous
A major concern with the selective COX-2 inhibitors has acid, HOBr (from bromide ions; though this appears to be
been the increased tendency to thrombosis which has been a relatively minor product) (Chapman et al. 2009; Morgan
correlated with their lack of anti-platelet effects and et al. 2011) and hypothiocyanous acid, HOSCN, from
decreased synthesis of prostacyclin. This toxicity led to thiocyanate ions. The latter is potentially of major
the withdrawal of the selective COX-2 inhibitor, rofec- importance as markedly elevated levels of the precursor
oxib. As discussed above, paracetamol appears to be a ion, thiocyanate (SCN-), are present in some subjects,
selective COX-2 inhibitor and it has been suggested that particularly smokers and those on particular diets, and this
paracetamol may also show a similar pattern of adverse ion is the preferred substrate for the enzyme. Unlike
effects on the vascular system (Hinz et al. 2008; Hinz and HOCl and HOBr, which are relatively promiscuous oxi-
Brune 2012). The effect of paracetamol on blood pressure dants that damage multiple targets, HOSCN is a highly
is unclear (Table 1; see Therapeutic and toxic actions selective oxidant of thiol groups and has been shown to
similarities and differences from NSAIDs section). have marked effects of cellular redox balance, cell sig-
Major cardiovascular events may be increased by para- nalling processes and apoptosis (Lloyd et al. 2008; Lane
cetamol although the effect was only seen in smokers et al. 2010).
(Chan et al. 2006). Of more concern is the finding from a Nitrite is an anion which, like chloride, bromide and
large epidemiological study that paracetamol increases the thiocyanate, may be involved in inflammatory processes.
risk of preeclampsia (Rebordosa et al. 2010). Again, this Nitrite is readily oxidised by compound I of myeloperox-
was suggested to be due to the imbalance in the prosta- idase (Burner et al. 2000; Fig. 4) and converts tyrosine
cyclin and thromboxane A2 caused by the COX-2 residues of proteins to nitro tyrosine residues (Van der
selectivity of paracetamol. Confirmation of this finding on Vliet et al. 1997). As outlined below, paracetamol inhibits
pregnancy is, however, required. the oxidation of low-density lipoproteins by myeloperoxi-
Inhibition of myeloperoxidase and other peroxidases dase in the presence of nitrite. This effect of paracetamol
may largely overcome or reverse the effects of paracetamol may also be a significant aspect of its effects on inflam-
on blood pressure and thrombosis due to selective inhibi- matory processes.
tion of COX-2. The production of HOCl is a mechanism of the anti-
bacterial and anti-fungal actions of neutrophils, but other
Inhibition of myeloperoxidase pathways are also important and inhibition of myeloper-
oxidase is unlikely to increase the likelihood of infections
As well as being an inhibitor of COX-1 and COX-2 (Rosen and Michel 1997). This suggestion is supported by
through an action on its associated peroxidase function, findings in people with naturally occurring MPO deficiency
paracetamol is also a substrate and inhibitor of myeloper- that show no effect on MPO deficiency on bacterial
oxidase, an enzyme in neutrophils and to a lesser extent in infection rates. However, the IC50 for paracetamol-induced
monocytes (Figs. 4, 5). Myeloperoxidase catalyses the inhibition of MPO is approximately 80 lM compared with

123
The modern pharmacology of paracetamol 219

plasma concentrations fluctuating from about 13 to Potential novel clinical effects of paracetamol
200 lM (see Chemistry and distribution section).
Consequently, inhibition of MPO by paracetamol appears Effects of paracetamol on osteoarthritis and rheumatoid
only partial and it is unlikely that infections will be arthritis
increased by paracetamol. There has been concern that
antipyretics decrease immunological function but para- Myeloperoxidase may be associated with the development
cetamol does not appear to change the course of severe of osteoarthritis. In early, although not in late osteoarthritis,
gram-negative sepsis (Mohr et al. 2012). Furthermore, there are elevated levels of myeloperoxidase and chlori-
paracetamol does not interfere significantly with bacterial nated proteins in synovial fluid (Steinbeck et al. 2007).
killing by amoxycillin or cephalosporins in serum Paracetamol has been linked to decreased volumes of
(Carsenti-Etesse et al. 1998). synovial fluid in osteoarthritis, an indication of an anti-
inflammatory effect, but more detailed clinical evaluation
Potential effects of paracetamol on factors involved is required (Table 3). Very large numbers of neutrophils
in atherogenesis are present in the synovial fluid of patients with rheumatoid
arthritis with the possibility of inflammatory effects from
Inhibition of myeloperoxidase by paracetamol may be of the myeloperoxidase in these cells.
particular importance in decreasing the oxidation of both It has been suggested that inhibition of myeloperoxidase
low- and high-density lipoproteins by HOCl. Oxidation of could be a site of the action of thiol drugs, such as peni-
low-density lipoproteins is considered to be pro-athero- cillamine, in the treatment of rheumatoid arthritis (Cuperus
genic while damage to high-density lipoproteins may et al. 1985). Although penicillamine is not widely used
diminish their anti-atherogenic actions (Malle et al. 2006a, now for the treatment of rheumatoid arthritis, inhibition of
b). Compelling evidence has also been presented for the myeloperoxidase by paracetamol could provide long-term
presence of myeloperoxidase in the subendothelial matrix benefit in rheumatoid arthritis. This should be tested.
of blood vessels (Baldus et al. 2001) in advanced athero- Inflammation is a feature common to both rheumatoid
sclerotic lesions that are liable to rupture (Daugherty et al. arthritis and atherosclerosis, and inhibition of both by
1994; Sugiyama et al. 2001) and may be a mediator of paracetamol may have clinical potential.
pathological formation of neointima in atherosclerosis
(Yang et al. 2006). More directly, low doses of paracetamol Inhibition of ischaemiareperfusion injury
reduce the thickness and amount of oxidised protein in rat
aorta (Rice et al. 2012) and have also been reported to An exciting new aspect of the pharmacology of paraceta-
decrease fatty streaks in the aortas of rabbits fed 1 % mol is that it may decrease ischaemia/reperfusion injury.
cholesterol although only abstracts of this latter work have Reperfusion of ischaemic tissues results in increased for-
been presented (Taylor et al. 1999). mation of oxidising free radicals that lead to tissue damage.
Paracetamol inhibits the myeloperoxidase-induced Several studies have shown that paracetamol decreases
chlorination of the amino groups of heparan sulphate and tissue damage after reperfusion in experimental animals.
extracellular matrix (Koelsch et al. 2010). Heparan sul- The tissue damage that is decreased by paracetamol
phate is a major component of the basement membrane includes cerebral mitochondrial dysfunction (Baliga et al.
of blood vessels and its chlorination and degradation may 2010, 2011), arrhythmias (Merrill et al. 2007) and myo-
be important in the development of atherosclerosis and cardial damage (Merrill et al. 2004; Zhu et al. 2006) after
kidney disease (Nicholls and Hazen 2005; Malle et al. reperfusion. However, experimental studies are still
2003). Therapeutic concentrations of paracetamol also inconsistent and no cardioprotective effect was found by
decrease the nitrite-induced oxidation of low-density Leshnower et al. (2006) in sheep and rabbits.
lipoproteins by myeloperoxidase (Chou and Greenspan A novel effect of paracetamol is its inhibition of the up-
2002) and monocytes in vitro (Nenseter et al. 1995) and regulation of osteopontin in the right cortex following
in vivo in experimental animals (Ozsoy and Pabuccuoglu ischaemiareperfusion of the brain (Baliga et al. 2011).
2007). Osteopontin is a factor associated with several inflamma-
Overall, the cardiovascular effects of myeloperoxidase tory responses including reperfusion injury. It is significant
appear significant in the development of atherosclerotic that these effects were produced at therapeutic doses
lesions and, consequently, inhibition of myeloperoxidase (515 mg/kg). The data are, however, conflicting with an
by paracetamol could slow the development of athero- absence of activity of paracetamol in some ischaemia/
sclerosis. There is sufficient evidence to indicate that reperfusion studies (Rork et al. 2006).
paracetamol should be evaluated carefully in clinical trials An activity of paracetamol analogous to its actions on
on atherosclerosis. ischaemiareperfusion injury may be its attenuation of

123
220 G. G. Graham et al.

neuronal cell death induced by menadione which is reported potency is very variable with IC50 values: from
described as a superoxide releasing oxidant stressor very low therapeutic levels of 13 lM (Molnar and Garai
(Tripathy and Grammas 2009). Pretreatment of neuronal 2005) to grossly supratherapeutic concentrations of 10 mM
cultures with paracetamol (25300 lM) increases the sur- (Senter et al. 2002). The hepatotoxic metabolite of para-
vival of neurones and decreases the secretion of several cetamol, NAPQI, is an inhibitor of the tautomerase with a
inflammatory cytokines in cortical cultures exposed to quite a high IC50 of 46 lM (Senter et al. 2002). NAPQI
menadione. does, however, decrease the immunoreactivity and cellular
The mechanism of these positive actions of paracetamol binding of MIF (Senter et al. 2002). Interestingly, when
is uncertain but a possibility is inhibition of myeloperoxi- MIF is cocrystallized with NAPQI, the NAPQI is converted
dase. Despite the conflicting results, there are sufficient to a paracetamol dimer (Fig. 5) which binds reversibly to
positive results for paracetamol to be evaluated in limiting MIF (Crichlow et al. 2009).
stroke and myocardial infarction. The interactions between paracetamol and MIF are
unclear but a significant interaction at therapeutic con-
Attenuation of rhabdomyolysis centrations could lead to clinically important effects on
rheumatoid arthritis, atherosclerosis and diabetes.
Rhabdomyolysis is due to muscle injury which releases
myoglobin into the circulation resulting in acute renal Antidiabetic actions
impairment which can be very severe. The mechanism of
the renal damage is thought to be redox cycling of the Paracetamol has antidiabetic actions in diabetes in mice
heme group in myoglobin resulting in the oxidation of produced by streptozotocin and high-fat diets (Shertzer
lipoproteins and vasoconstriction. A recent study showed et al. 2008; Kendig et al. 2008). Paracetamol also decreases
that the renal damage in a rat model of rhabdomyolysis is the increase in body fat produced by the high-fat diet or
decreased by paracetamol (Boutaud et al. 2010). The mode olanzapine in mice (Kendig et al. 2008; Shertzer et al.
of action of paracetamol in this situation appears to be 2010). The non-selective NSAIDs produce the same
similar to its inhibition of myeloperoxidase and the per- effects, indicating that these effects are produced by inhi-
oxide functions of COX-1 and COX-2, involving the bition of the synthesis of PGs and related factors (Kendig
oxidation of paracetamol to free radicals and dimers et al. 2008; Shertzer et al. 2008). Salicylate also has some
(Fig. 4; Boutaud and Roberts 2011). In view of the safety antidiabetic effects in man and is a further indication that
of paracetamol and the danger of rhabdomyolysis, it would COX inhibition causes the reduction in blood glucose
seem advantageous to test the usefulness of paracetamol in (Desouza 2010).
this condition. It is significant that the antidiabetic actions of paracet-
amol are produced at relatively low doses (2030 mg/kg
Inhibition of macrophage migration inhibitory factor daily) in mice, an indication of potential clinical effects in
(MIF) man.

MIF is considered to be an important cytokine in the Adaptations of muscle and tendon

development of rheumatoid arthritis and atherosclerosis
and possibly also in diabetes and cancer. This cytokine also Recent studies have shown surprising effects of paraceta-
has enzymatic activities of ketoenol isomerism (tautom- mol on muscle and tendon when its dosage was concurrent
erase) (Fig. 7; Rosengren et al. 1997) and, possibly also, with resistance training of the quadriceps femoris muscle in
thiol-protein oxidoreductase activities (Kleemann et al. adults aged more than 60 years. Treatment with paraceta-
1998). The significance of the tautomerase activity is mol (4 g daily) together with resistance training of the
unclear but this enzymatic action may be required for quadriceps produced greater increases in muscle volume
proteinprotein interactions (Conroy et al. 2010). Paracet- and strength than training alone (Trappe et al. 2011). The
amol is an inhibitor of the tautomerase activity but the diameter of the patellar tendon also increased and there

Fig. 7 Ketoenol tautomerism

of phenylpyruvic acid catalysed
by macrophage migration
inhibitory factor (MIF)

123
The modern pharmacology of paracetamol 221

were also changes in the mechanical properties of the pain (Nielsen et al. 1992). The faster absorption of para-
tendon (Carroll et al. 2011). Ibuprofen produced similar cetamol when administered with caffeine is associated with
changes to paracetamol in the muscle but not in the patellar greater overall analgesic activity of paracetamol (Renner
tendon. The clinical significance of these changes is et al. 2007). On the other hand, slower absorption has been
unclear but requires further study, particularly considering associated with lesser initial analgesic effects, but more
the wide use of paracetamol and the value of exercise in the prolonged analgesia in other studies (Mller et al. 2000;
treatment of osteoarthritis. Strom et al. 1990).
Concerns about the rate of absorption of paracetamol are
significant for the formulation of sustained-release para-
Cancer cetamol. Presently marketed sustained-release tablets of
paracetamol contain both immediate and sustained release
Unlike the considerable amount of work showing that layers. Although the data on the effect of rate of absorption
aspirin and the selective COX-2 inhibitors have potential are conflicting, it still seems optimal to combine fast and
value in the prevention and treatment of colon and other slow absorption components in the one tablet.
cancers, there has been little corresponding work on par-
acetamol. However, paracetamol may decrease the
incidence of prostate cancer (Jacobs et al. 2011) although it Adverse effects
may increase the rate of acute leukaemia and multiple
myeloma (Robak et al. 2008). Conflicting influences of The excellent tolerance, particularly the gastrointestinal
paracetamol on ovarian cancer have been reported with a tolerance of therapeutic doses of paracetamol, is a major
decreased rate (Bonovas et al. 2006; Deffieux et al. 2007) reason for its recommendation and widespread acceptance
whereas two studies have found no significant association as an analgesic.
with ovarian or endometrial cancer (Setiawan et al. 2012;
Neill et al. 2012). Experimentally, paracetamol protected
against induced colon cancer in rats (Williams et al. 2002). Gastrointestinal tolerance

Paracetamol is generally tolerated well by the gastroin-

Formulations of paracetamol testinal tract (Latimer et al. 2009) This is in contrast to the
well-known adverse effects of the non-selective NSAIDs.
Several formulations of paracetamol are available including The introduction of the selective COX-2 inhibitors
plain, effervescent and sustained release tablets, drops for improved gastrointestinal tolerance but some adverse
children, suppositories and intravenous preparations. An effects on the upper gastrointestinal tract remain (see
injectable glycine ester of paracetamol, propacetamol, was Therapeutic and toxic actions: similarities and differences
used but has been replaced with a simple solution of para- from NSAIDs section; Latimer et al. 2009).
cetamol. The paracetamol in the presently available The gastrointestinal tolerance of combinations of para-
formulation contains 1 g paracetamol in 100 mL and can only cetamol and NSAIDs is more questionable. Large-scale
be used by short intravenous infusion as the volume is too clinical and epidemiological studies on paracetamol and non-
large to be administered intramuscularly or subcutaneously. selective NSAIDs indicate that the combination of paraceta-
Levy (1980), a notable pioneer in pharmacokinetic mol and a non-selective NSAID may produce greater
research, contends that rapid absorption leads to a greater gastrointestinal toxicity than either drug alone (Doherty et al.
continuing analgesia of non-narcotic analgesics. According to 2011; Rahme et al. 2008). In experimental animals, paracet-
this hypothesis, greater initial analgesia due to faster absorp- amol (100 mg/kg) produces no significant gastric damage but
tion leads to increased analgesia for several hours. Consistent the combination of paracetamol with meloxicam (0.9 mg/kg)
with this hypothesis, greater continuing analgesia has been or ibuprofen (100 mg/kg) produces greater gastric damage
produced by intravenous paracetamol than oral paracetamol in than meloxicam or ibuprofen alone (Kumar et al. 2010).
at least one study (Jarde and Boccard 1997) although no such Further work on the possible adverse gastrointestinal effects
contrast was observed in another study (Moller et al. 2005). of combinations of paracetamol and NSAIDs is required.
There are conflicting data on the time course of anal-
gesic activities of oral formulations of paracetamol. In
agreement with Levys hypothesis, no significant analgesia Hepatotoxicity
was seen after 2 g of sustained-release paracetamol
although 0.5 and 1 g doses of immediate-release paracet- Overdoses of paracetamol are well known to cause major
amol showed analgesic activity in tests of experimental hepatotoxicity which may progress to death if the dose is

123
222 G. G. Graham et al.

sufficient and the patients are not treated adequately with Overall, the risk of severe hepatotoxicity from thera-
N-acetylcysteine. Paracetamol poisoning is now the most peutic doses appears extremely low.
common cause of acute liver failure in the United States
and the United Kingdom. FDA decision on dosage
The hepatotoxicity of paracetamol overdoses is due to
the formation of the oxidised metabolite of paracetamol The United States FDA recently limited the paracetamol
and its reaction with glutathione (Fig. 5). Glutathione, in its content of prescription tablets to 325 mg, making the total
reduced form, maintains the appropriate redox balance in daily dose of paracetamol 2,600 mg if eight tablets are
cells and prevents cell death. Centrilobular necrosis occurs taken daily.
because of depletion of glutathione and also because, after
depletion of hepatocellular glutathione, the oxidised para- Unintentional versus intentional overdose
cetamol metabolite reacts with essential cellular proteins.
A variety of factors affect the hepatotoxicity of paraceta- In the UK, a great majority of overdoses are intentional
mol but, apart from the contentious area of hepatotoxicity (Makin and Williams 2000; Craig et al. 2011). There is
of therapeutic doses of paracetamol, are not reviewed in conflicting data on the causes of hepatotoxicity in USA. In
this communication. one survey of severe overdose from USA, 48 % reported
unintentional overdosage with 44 % overdoses taken with
Hepatotoxicity from therapeutic doses of paracetamol suicidal intent (Larson et al. 2005). However, in two other
surveys, approximately 70 % of overdoses were reported as
Occasional hepatotoxicity is claimed widely but critical intentional (Schiodt et al. 1997; Yarema et al. 2010). The
examination of cases shows that most patients whose tox- numbers of unintentional overdoses have increased at a
icity is claimed from therapeutic doses have probably taken greater rate than intentional overdoses (Yarema et al. 2010).
overdoses (Prescott 2000a; Graham et al. 2005). A back- The reasons for the high rate of unintentional overdose
ground presentation to a FDA conference on paracetamol in USA are uncertain but, almost certainly, a contributing
and hepatotoxicity contained the statement that rare cases factor is the very high usage of combination tablets of
of acute liver injury have been linked to amounts lower paracetamol (presently 500 mg, to be reduced to 325 mg)
than 2.5 g/day but there was no comment on the difficulty with the potent opiates, hydrocodone or oxycodone. A
in assigning hepatotoxicity to therapeutic doses of para- recent survey in the USA indicates that even the prescribed
cetamol (FDA Background 2009), as has been noted from a dose of combination tablets yields more than the maximum
large USA survey (Larson et al. 2005). paracetamol dose of 4 g daily in 8 % of all prescriptions
Re-challenge of patients with hepatotoxicity from (Mort et al. 2011). Further, most patients (63 %) with
claimed therapeutic dosage is extremely uncommon. unintentional overdose were taking the combination of
However, three patients with hepatotoxicity have been re- paracetamol and an opioid (Larson et al. 2005). Excessive
challenged with a rapid rise in transaminase concentrations dosage may be due not only to high prescribed doses but
in plasma detected (Graham and Scott 2005). Elevated also to worsening pain leading to increased self-adminis-
plasma transaminases are also noted in some young patients tration of both plain and combination tablets of
during treatment with therapeutic doses of paracetamol paracetamol and opioids (Larson et al. 2005). Continued
(Watkins et al. 2006), but the high transaminase levels have dosage of the opioids may increase the development of
declined or reverted to normal over time despite continuing tolerance to the opioid with the subsequent increase in the
dosage (Kuffner et al. 2006). An important recent obser- dosage of the opioid and a greater subsequent dose of
vation is the comparative levels of alanine aminotransferase paracetamol.
in hospitalised patients taking paracetamol. The levels of The FDA requirement for a lower dose of paracetamol in
alanine aminotransferase were similar in older and younger the combination tablets is designed largely to reduce the
patients despite higher plasma concentrations of paraceta- problem of unintentional overdose although intentional
mol in the older patients (Mitchell et al. 2011). A further overdosage may also be reduced. A more long-term
indication of the safety of therapeutic doses of paracetamol improvement may be to restrict greatly the use of the full
is that serious hepatotoxicity has never been recorded in opioid/paracetamol combinations or to separate hydroco-
prospective clinical trials on 30,865 patients (Dart and done and oxycodone from the paracetamol (Larson et al.
Bailey 2007). Admittedly, prospective trials are conducted 2005).
in controlled conditions and patients with complex medical The over-the-counter (OTC) sales of paracetamol are
histories are often excluded. Nevertheless, the absence of now restricted to small numbers of paracetamol tablets in
any serious hepatotoxicity in clinical trials is a strong blister packs in UK and many other countries. Surveys in
indicator of the safety of paracetamol. England and Wales found that there were 765 fewer deaths

123
The modern pharmacology of paracetamol 223

(43 % reduction) in the 11 years since the introduction of directly, the metabolism of paracetamol is not changed in
the small blister packs (Hawton et al. 2013). The causes of obese patients during food restriction (Schenker et al.
this reduced severe poisoning from paracetamol are still 2001).
unclear but the uncontrolled availability of packs contain-
ing large numbers of plain paracetamol (up to 1,000) in
USA is a clear danger for overdoses, particularly for chil- Interactions
dren (Graham et al. 2010).
A feature of the clinical use of paracetamol is its small
Alcohol and the risk of hepatotoxicity number of interactions with other drugs although, not
surprisingly, opioids and NSAIDs, potentiate the analgesic
The role of alcohol in the hepatotoxicity of paracetamol is actions of paracetamol (see Clinical analgesic efficacy of
contentious. Chronic alcohol abuse is recorded in about paracetamol section).
50 % patients who have taken overdoses of paracetamol, Apart from the mutual potentiation with other analge-
whether these are unintentional or intentional (Makin and sics, paracetamol also can potentiate the anticoagulant
Williams 2000; Larson et al. 2005; Craig et al. 2011). effects of warfarin, and clotting tests should be conducted
Many alcoholics claim that they have taken only thera- if paracetamol treatment is added or removed from the
peutic doses of paracetamol, but obtaining reliable histories treatment of patients taking warfarin. Paracetamol does not
from these patients is notoriously difficult and the ingestion have sufficient anti-platelet effect to potentiate the antico-
of no more than therapeutic doses in alcoholic subjects is agulant effect of warfarin and the interaction is due to
almost certainly grossly overstated (Prescott 2000b; Dart paracetamol decreasing the synthesis of vitamin K-depen-
et al. 2000). A recent prospective trial indicated that the dent clotting factors (Mahe et al. 2006).
hepatotoxicity of therapeutic doses of paracetamol is not There has been much concern about the potential effects
increased in newly abstinent alcoholic patients who should of other drugs, including alcohol, on the oxidative metab-
have been most sensitive to the hepatotoxicity of paracet- olism and hepatotoxicity of paracetamol. Substantial
amol due to up-regulation of oxidising cytochrome P450 hepatotoxicity from therapeutic doses of paracetamol
systems (Dart et al. 2010). appears unlikely (see Hepatotoxicity from therapeutic
Although evidence for a causal relationship between doses of paracetamol section), but a proven interaction is
alcohol and paracetamol-induced hepatotoxicity is weak the extended half-life of paracetamol when taken with
(Graham et al. 2005), the FDA has required all formulations probenecid (Abernethy et al. 1985). The dosage of para-
containing paracetamol to be labelled with an alcohol cetamol should then be decreased to a maximum of 3 g
warning, namely that the patient should ask their doctor if daily.
they should take paracetamol if they consume three or more
alcoholic drinks per day. As far as we are aware, this
labelling is not required in other countries. Comparative activities of paracetamol and other
phenols
Hepatotoxicity and malnutrition
The literature now contains many studies on plant pheno-
Several case studies of hepatotoxicity have included lics with similar overall pharmacology to paracetamol. An
statements that patients have developed hepatotoxicity example is the well-known complex phenol and potential
from therapeutic doses of paracetamol when they have not anti-cancer agent, resveratrol (Fig. 8), which has similar
been eating well or fasting. Causal relationships are, pharmacological activities to paracetamol (Table 6). Fur-
however, difficult to establish (Lauterburg 2002). More ther work is, however, required to compare resveratrol and

Table 6 Some
Pharmacological action References
pharmacological properties of
resveratrol which are also Centrally mediated analgesia in rats Falchi et al. (2010)
shown by paracetamol
Antipyresis Sebai et al. (2009)
Oxidation by myeloperoxidase and inhibition of HOCl production Kohnen et al. (2007)
Inhibition of prostaglandin synthesis in vitro Wendeburg et al. (2009)
Inhibition of up-regulation of osteopontin Sutra et al. (2008)
Inhibition of tautomerism activity of MIF Molnar and Garai (2005)

123
224 G. G. Graham et al.

The mechanism of action of paracetamol is clearly

linked to PG pathways and the consequent interaction with
other pain pathways. New indications for its use as an
antioxidant are being investigated, particularly effects
related to inhibition of myeloperoxidase.

Fig. 8 Structure of resveratrol

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