Dr. Satyajit Panda (Asst.
Prof,)
IPT, Salipur
POSOLOGY
POSOLOGY is derived from two Greek words ‘posos’ meaning ‘how much’ and ‘logos’
meaning ‘science’. In pharmaceutical science when we talk regarding ‘how much or quantity’,
it indicates dose, because dose is defined as the quantity of drug to be taken at once to elicit the
desired therapeutic effect. So posology is the branch of medical science that deals with doses.
The optimum dose of a drug varies from patient to patient. Several factors are responsible for
this variance. Some of these factors that influence the dose of a drug are described below.
1. Age: Human beings can be categorized into the following age groups:
1. Neonate: From birth up to 30days.
2. Infant: Up to 1 year age
3. Child in between 1 to 4 years
4. Child in between 5 to 12 years.
5. Adult
6. Geriatric (elderly) patients
In children the enzyme systems in the liver and renal excretion remain less developed. So, all
the doses must have to be less than that of an adult dose. In elderly patients the renal functions
decline. Metabolism rate in the liver also decreases. Drug absorption from the intestine becomes
slower in elderly patients. So in geriatric patients the dose is less and should be judiciously
administered.
2. Sex: Women don’t always respond to the action of the drugs in same manner as men. Special
care should be taken while administering any drug to the women during menstruation,
pregnancy and lactation (breast feeding). Strong purgatives should not be given in
menstruation and pregnancy. Antimalarials, ergot alkaloids should not be taken during
pregnancy to avoid deformation of foetus. Antihistaminic and sedative drugs are not taken
during breast feeding because these drugs are secreted in the milk and the child may consume
them.
3. Body size: It influences the concentration of drug in the body. The average adult dose is
calculated for a person with 70kg body weight (BW). For exceptionally obese (fat) or lean (thin)
patient the dose may be calculated on body weight basis.
BodyWeight(kg )
Individual dose = x Average adult dose
70
Another method of dose calculation is according to the body surface area (BSA). This method is
more accurate than the body weight method.
Dr. Satyajit Panda (Asst. Prof,)
IPT, Salipur
Body surface area (m 2 )
Individual dose = x Average adult dose
1.7
The body surface area (BSA) of an individual can be obtained from the following formula:
BSA (m2) = BW (kg)0.425 x Height (cm)0.725 x 0.007184
4. Route of administration
In case of intravenous injection the total drugs reaches immediately to the systemic circulation
hence the dose is less in i.v. injection than through oral route or any other route.
5. Time of administration
The drugs are most quickly absorbed from empty stomach. The presence of food in the
stomach delays the absorption of drugs. Hence a potent drug is given before meal. Irritant drug
is given after meal so that it will be diluted with food and thus produce less irritation.
6. Environmental factors
Stimulant types of drug are taken at day time and sedative types of drugs are taken at night. So
the dose of a sedative required will be much higher in day time than at night. Alcohol is better
tolerated in winter than in summer.
7. Psychological state
Sometimes personality and behavior of a physician may influence the effect of the drug.
Females are more emotional than males, hence requires less dose of the drugs. Psychological
state of mind can affect the response of a drug, e.g. a nervous and anxious patient requires
more general anaesthetics. Placebo is an inert substance that does not contain any drug.
Commonly used placebos are lactose tablets and distilled water injections. Some time patients
often get some psychological effects from this placebo. Placebos are more often used in clinical
trials of drugs.
8. Pathological states (i.e. Presence of disease)
Several diseases may affect the dose of drugs:
In gastrointestinal disease like achlorhydria (reduced secretion of HCl acid in the stomach) the
absorption of aspirin decreases.
In liver disease (like liver cirrhosis) metabolism of some drugs (like morphine, pentobarbitone
etc.) decreases.
In kidney diseases excretion of drugs (like aminoglycosides, digoxin, phenobarbitone) are
reduced, so less dose of the drugs should be administered.
9. Accumulation
Dr. Satyajit Panda (Asst. Prof,)
IPT, Salipur
Any drug will accumulate in the body if the rate of absorption is more than the rate of
elimination. Slowly eliminated drugs are often accumulated in the body and often causes
toxicity e.g. prolonged use of chloroquine causes damage to retina. So dosing frequency should
be reduced.
10. Drug interactions
Simultaneous administration of two drugs may result in same or increased or decrease effects.
Drug administration with Pharmacological
dose effect
Drug A Effect A
Drug B Effect B
Drug A + Drug B Effect AB
Relationship Name of the effect Examples
Effect AB = Effect A + Effect B Additive effect Aspirin + Paracetamol
Effect AB > Effect A + Effect B Synergism (or potentiation) Sulfamethaxazole +
Trimethoprim
Effect AB < Effect A + Effect B Antagonism Histamine + Adrenaline,
Adrenaline + Acetylcholine
11. Idiosyncrasy (Unusual, unexpected drug effects)
Some persons may produce abnormal reactions of the drug in the amount of standard dose.
This unusual reaction is termed as Idiosyncrasy. This is an exceptional response to a drug in few
individual patients. For example, in some patients, aspirin may cause gastric haemorrhage,
penicillin causes irritating rashes on the skin etc.
12. Genetic diseases
Some patients may have genetic defects. They lacks with some enzymes. In those cases some
drugs are contraindicated.
e.g. Patients lacking Glucose-6-phosphate dehydrogenase enzyme should not be given
primaquine (an antimalarial drug) because it will cause haemolysis.
13. Tolerance
Some time higher dose of a drug is required to produce a given response (previously less dose
was required).
Natural Tolerance: Some races are inherently less sensitive to some drugs, e.g. rabbits and black
races (Africans) are more tolerant to atropine.
Acquired Tolerance: Repeated use of a drug in an individual for a long time may require larger
dose to produce the same effect, which was obtained with normal dose previously.
Dr. Satyajit Panda (Asst. Prof,)
IPT, Salipur
Cross tolerance: It is the development of tolerance to pharmacologically related drugs e.g.
alcoholics are relatively more tolerant to sedative drugs.
Tachyphylaxis: (Tachy = fast, phylaxis = protection) is rapid development of tolerance. When
doses of a drug are repeated in quick succession a reduction in response occurs – this is called
tachyphylaxis. This is usually seen in ephedrine, nicotine.
Drug resistance: It refers to tolerance of microorganisms to inhibitory action of antimicrobials
e.g. Staphylococci to penicillin.
CALCULATIONS OF DOSES FOR CHILDREN
A number of methods have been used to relate doses for children to their ages.
1. Dose proportionate to age
Young’s formula: This formula is used for children having age below 12 years.
Age in years
Dose for the child = x Adult dose
Age + 12
Dilling’s formula: This formula is used for children having age from 4 to 20 years. This formula
is better because it is easier to calculate the dose.
Age in years
Dose for the child = x Adult dose
20
Age in years + 1
Dose for the child = x Adult dose
Cowling’s formula (4-12years): 24
Freud’s formula: For less than 12 years of age
Age in months
Dose for the child = x Adult dose
150
2. Doses proportionate to body weight
Weight in pound
Clark’s formula: Dose for the child = x Adult dose
150
Weight in kg
Dose for the child = x Adult dose
68 .2
3. Doses proportionate to body surface area (BSA)
Body surface area of the child (m 2 )
Dose of a child = x Average adult dose
1.7
Dr. Satyajit Panda (Asst. Prof,)
IPT, Salipur
TABLE: Calculation of child doses
Fraction of adult dose
Age Weight (kg) Height (cm) BSA (m2) Young’s Clark’s BSA
rule Rule method
Birth 3.5 50.5 0.21 – 0.05 0.12
3 mos 5.7 59.9 0.29 0.02 0.08 0.17
6 mos 7.5 65.8 0.35 0.04 0.11 0.20
1 yr 9.9 74.7 0.44 0.08 0.15 0.25
2 yrs 12.5 86.9 0.54 0.14 0.18 0.31
3 yrs 14.5 96.0 0.61 0.20 0.21 0.35
4 yrs 16.5 103.4 0.68 0.25 0.24 0.39
5 yrs 19.1 110.5 0.76 0.29 0.28 0.44
6 yrs 21.5 116.8 0.84 0.33 0.32 0.49
7 yrs 24.2 123.2 0.91 0.37 0.35 0.53
8 yrs 26.9 129.0 0.98 0.40 0.39 0.57
9 yrs 29.5 134.1 1.04 0.43 0.43 0.60
10 yrs 32.3 139.4 1.12 0.45 0.47 0.65
11 yrs 35.5 144.5 1.20 0.48 0.52 0.69
12 yrs 39.1 150.9 1.28 0.60 0.57 0.74
Exerxise: What will be the dose for a child of 6 years if the adult dose is 500mg.
