SEM.

I, PHARMACEUTICS I 1

POSOLOGY

‘Posology’ is a branch of medical science which deals with dose or quantity of drugs which can be
administered to a patient to get desired pharmacological and therapeutic action.
The word ‘Posology’ is derived from the Greek words ‘Posos’ meaning ‘how much’ and ‘Logos’
meaning ‘science’.

Factors Affecting Dose of a Drug/ Factors Affecting Posology:-

1) Age:
 The age of an individual is important, especially in the case of young or very old patients.
 Children and old patients need lesser amount of drug than the standard adult dose because, they are unable
to degrade or excrete drugs as that of adults.
 Children can tolerate relatively large doses of digitalis, ethanol but elderly patients are more sensitive to
some drugs.
 Some drugs may cause a state of confusion in some patients.
 Newborn infants are abnormally sensitive to certain drugs due to their immature state of hepatic and renal
function.
2) Body weight:
 Generally recommended adult doses are based on a normal body weight of 70kg.
 But such a dose will be too less for a muscular person weighing 100kg and too large for a weak person
weighing about 50kg.
 The calculation of doses for children on the basis of body weight is considered more dependable than based
on age.
 The doses calculated according to body weights are expressed as mg/kg body weight.
 The dose pattern mentioned above may not apply in case of obese patients, children and malnourished
patients.
3) Sex:
 Generally, females require lesser dose than males due to their lesser weight and they are more responsive to
the effects of certain drugs than males.
 Drugs should be given very carefully during menstruation, pregnancy and lactation.
 Strong purgative drugs should be avoided during menstruation.
 Drugs which stimulate contraction of uterus like antimalarial drugs, should be avoided in pregnant ladies
which may lead to abortion or miscarriage.
 Drugs like alcohol, barbiturates, narcotic and non-narcotic analgesics, etc., which get transported from
mother to the fetal circulation should be avoided.
 These drugs produce adverse effects on the fetus, sometimes, may result into death of the fetus in the uterus.
 During lactation, drugs like morphine, tetracycline, etc. which are excreted in milk should be given very
carefully to the mother.
4) Route of administration:
 The dose of a given drug may vary according to the dosage form and route of administration used.
 Drugs administered intravenously enter the blood stream directly, hence require lesser dose than the
subcutaneous dose which in turn is smaller than the oral dose.
 The onset of action of IV medicines is faster and this enhances the chances of drug toxicity.
5) Time of administration:

TINA RAJU, ASSOCIATE PROFESSOR, DR. MOOPEN’S COP, WAYANAD

 SEM. I, PHARMACEUTICS I 2

 The presence of food in the stomach delays the absorption of drugs.

 Drugs are rapidly absorbed from the empty stomach.
 So, the amount of drug that are effective when taken before a meal may be ineffective if administered during
or after meals.
 Irritating drugs are better tolerated if administered after meals which will dilute the drugs concentration and
reduce gastric irritation.
 E.g.:- Iron, Arsenic, Cod liver oil, etc.
6) Presence of disease:
 Drugs are more effective in diseased conditions than normal body conditions.
 During fever, one can tolerate high doses of antipyretic.
 During hepatic or renal diseases, the drugs which are metabolized in the liver or excreted through the
kidneys may prove fatal.
 Drugs like barbiturates and chlorpromazine may produce harmful effects in patients with liver cirrhosis.
 Streptomycin is excreted through kidney and is toxic if the patient has kidney failure.
7) Environmental Factors:
 Alcohol is better tolerated in cold environments than in summer.
 Daylight is stimulant thus enhances the effect of stimuLating drugs and decreases the effect of hypnotics and
sedatives.
 Hypnotics and sedatives are more effective at night.
 Dose of a sedative required to produce sleep during day time is much more than the dose required during
night.
8) Emotional Factors:
 Females are more emotional and responsive to drugs, thus require less dose of drugs.
 Nervous patients require smaller doses of drugs as compared to normal patients.
 Psychological state of mind can affect the response of a drug, e.g. a nervous and anxious patient requires
more general anesthetics.
 Placebo is an inert substance that does not contain any drug.
 Commonly used placebos are lactose tablets and distilled water injections.
 Sometimes patients often get some psychological effect from placebos.
9) Accumulation:
 When a drug is repeatedly administered for a long time, it may accumulate in the body and produce toxic
effects.
 The drugs which produce such symptoms of poisoning are called cumulative drugs like digitalis, heavy
metals, etc.
 This is due to slow excretion, defective degradation or unexpectedly rapid absorption of drugs.
 But in case of some drugs like Chloroquine and Phenobarbitone this cumulative effect is desirable.
10) Synergism:
 When two or more drugs are used in combination, their action is either increased or decreased, depending
upon drugs used in the combination.
 When the potency or duration of action is increased, this phenomenon is called synergism.
 It is useful when desired therapeutic result is difficult to obtain with a single drug.
 The dose of individual drugs can be lowered.
 Synergism is of two types:

TINA RAJU, ASSOCIATE PROFESSOR, DR. MOOPEN’S COP, WAYANAD

 SEM. I, PHARMACEUTICS I 3

a) Addition- When the total effect of two drugs is just equal to the sum of their individual effects, it is
known as addition or additive effect.
b) Potentiation- When the combined effect of two drugs administered is greater than the sum of their
individual effects, it is known as potentiation.
11) Antagonism:
 When the action of one drug is opposed by the other drug, the phenomenon is known as antagonism.
 Effect of one drug is antagonized by the other.
 This is mainly helpful in poisoning cases.
 Milk of magnesia is given in acid poisoning where, alkaline effect of the milk of magnesia neutralizes the
effect of acid poisoning.
 Adrenaline and acetylcholine when given together neutralize the effect of each other because adrenaline is
vasoconstrictor and acetylcholine is vasodilator.
12) Habituation & Addiction:
Habituation-
 When repeated use of a drug leads to production of emotional or psychological dependence, this condition is
known as habituation.
 E.g.;- Coffee, Tea, Tobacco, etc.
 When such agents are withdrawn the individual can carry on his routine work.
 No physical dependence occurs.
Addiction-
 It is a state of physical and mental drug dependence.
 Continuous use of alcohol, opium, cocaine, heroin, morphine, etc. leads to addiction and turns the person
mad, who becomes a burden to society.
 The addicts are deeply attached to the drug and become slave of it.
 If he is unable to get the drug, he develops withdrawal symptoms which may lead to death.
 So, the drugs that produce addiction must be carefully prescribed.
13) Idiosyncrasy/ Hypersensitivity/ drug allergy:
 An extraordinary response to a drug which is different from its desired pharmacological action is known as
idiosyncrasy.
 All persons do not respond in a similar manner to same drug due to difference in susceptibility.
 Some may produce abnormal reaction.
 Such abnormal reactions are known as idiosyncrasy.
 This is due to frequent use of drugs like antibiotics, vitamins, etc.
 Once a person is sensitized, a minute dose will produce allergic reactions.
 Some persons are sensitive to penicillin and sulphonamides, so, they show allergic reactions.
 Small quantity of aspirin may cause gastric haemorrhage.
 Hypersensitivity is of two types:
a) Immediate type- It is serious and requires injection of adrenaline. It may lead to death.
b) Delayed type- Skin rashes or contact dermatitis may occur.
14) Tolerance:
 When a drug administered in an ordinary dose fails to produce the desired action and requires larger dose to
produce the same desired action, this phenomenon is said to be tolerance or drug resistance.
 E.g.; -Smokers can tolerate nicotine, alcoholics can tolerate large doses of alcohol, etc.
 It is of two types:

TINA RAJU, ASSOCIATE PROFESSOR, DR. MOOPEN’S COP, WAYANAD

 SEM. I, PHARMACEUTICS I 4

a) True tolerance- This is produced by oral and parenteral administration of drug.

b) Pseudo tolerance- This is produced only by oral administration of drug.
Other types of tolerance are as follows-
i. Natural Tolerance: Some races are inherently less sensitive to some drugs, e.g. rabbits and black race
(Africans) are more tolerant to atropine.
ii. Acquired Tolerance: By repeated use of a drug for a long time, a person may require larger dose of that drug
to produce the same effect that was obtained with normal dose previously.
iii. Cross tolerance: It is the development of tolerance to pharmacologically related drugs e.g. alcoholics are
relatively more tolerant to sedative drugs.
15) Tachyphylaxis:
 It is also known as acute tolerance.
 It is observed in some drugs, when they are administered repeatedly at very short intervals, the cell receptors
get blocked up and the response to that particular drug is decreased.
 By increasing the dose this cannot be reversed.
 But, if the administration of the drug is stopped for a long time and administered again after a gap of long
time, then the effect of the drug can be regained.
 E.g.; - Ephedrine, Cocaine, etc.
16) Metabolic disturbances:
 Changes in water electrolyte balance and acid-base balance, body temperature and other physiological
factors may modify the effects of drugs.
 The absorption of iron from G.I.T. is maximum if the patient has an iron deficiency anaemia.
 Salicylates reduce body temperature only when the temperature is high.

Formulae for calculation of doses:

a) Dose calculation according to age:
Young’s formula-
𝐴𝑔𝑒 𝑖𝑛 𝑦𝑒𝑎𝑟𝑠
Dose of a child= × 𝐴𝑑𝑢𝑙𝑡 𝑑𝑜𝑠𝑒
𝐴𝑔𝑒+12
This formula is used for calculating the doses of children less than 12 years of age.
Dilling’s formula-
𝐴𝑔𝑒 𝑖𝑛 𝑦𝑒𝑎𝑟𝑠
Dose of a child= × 𝐴𝑑𝑢𝑙𝑡 𝑑𝑜𝑠𝑒
20
This formula is used for calculating the doses of children between 4 and 20 years of age.
Cowling’s formula-
𝐴𝑔𝑒 𝑖𝑛 𝑦𝑒𝑎𝑟𝑠+1
Dose of a child= × 𝐴𝑑𝑢𝑙𝑡 𝑑𝑜𝑠𝑒
24
Fried’s formula-
𝐴𝑔𝑒 𝑖𝑛 𝑚𝑜𝑛𝑡ℎ𝑠
Dose of a child= × 𝐴𝑑𝑢𝑙𝑡 𝑑𝑜𝑠𝑒
150
This formula is used for calculating the doses of kids less than 2 years of age.
Bastedo’s formula-
𝐴𝑔𝑒 𝑖𝑛 𝑦𝑒𝑎𝑟𝑠+3
Dose of a child= × 𝐴𝑑𝑢𝑙𝑡 𝑑𝑜𝑠𝑒
30
Gaubin’s formula-
Fraction of adult dose for various ages is listed below:

TINA RAJU, ASSOCIATE PROFESSOR, DR. MOOPEN’S COP, WAYANAD

 SEM. I, PHARMACEUTICS I 5

Age (years) Dose with respect to adult dose

Under 1 1/12
1-2 1/8
2-3 1/6
3-4 ¼
4-7 1/3
7-14 ½
14-20 2/3
21-60 Full adult dose
60-70 4/5
70-80 ¾
Over 90 ½

b) Dose calculation according to body weight:

Clark’s formula-
𝑊𝑒𝑖𝑔ℎ𝑡 𝑖𝑛 𝑝𝑜𝑢𝑛𝑑𝑠
Dose of a child= × 𝐴𝑑𝑢𝑙𝑡 𝑑𝑜𝑠𝑒
150
OR
𝑊𝑒𝑖𝑔ℎ𝑡 𝑖𝑛 𝑘𝑔
Dose of a child= × 𝐴𝑑𝑢𝑙𝑡 𝑑𝑜𝑠𝑒
70

c) Dose calculation according to surface area:

𝐵𝑜𝑑𝑦 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑐ℎ𝑖𝑙𝑑
Dose of a child= × 𝐴𝑑𝑢𝑙𝑡 𝑑𝑜𝑠𝑒
𝐵𝑜𝑑𝑦 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑎𝑑𝑢𝑙𝑡
𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑐ℎ𝑖𝑙𝑑
Percentage of adult dose = × 100
𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑎𝑑𝑢𝑙𝑡
Dubois & Dubois formula for calculation of body surface area-
Body surface area = 0.20247 × height (m)0.725 × weight(kg)0.425
Mosteller formula for calculation of body surface area-
𝑤𝑒𝑖𝑔ℎ𝑡(𝑘𝑔)
Body surface area = √(ℎ𝑒𝑖𝑔ℎ𝑡 (𝑐𝑚) ×
3600
The average body surface area of an adult is 1.73 m2.
Age Percentage of adult dose
1month 10
2month 15
4month 20
1 year 25
3 years 35
5 years 40
10 years 60
12 years 75
16 years 90

TINA RAJU, ASSOCIATE PROFESSOR, DR. MOOPEN’S COP, WAYANAD

 SEM. I, PHARMACEUTICS I 6

QUESTIONS-
1. Define posology. Discuss the various factors which affect the dose of a drug in detail. 5M
1. Define posology. Explain the factors influencing dose of a drug. 5M
2. Define posology. 2M
3. Define the following terms: 2M
a) Tachyphylaxis b) Tolerance
c) Synergism d) Idiosyncrasy
4. How does age of patient have an impact on dose of a drug? 2M
5. What do you mean by cumulative drugs? 2M
6. What is the difference between synergism and antagonism? Give suitable example. 2M
7. What are the types of tolerance? 2M
8. How does body weight affect the dose of a drug? 2M
9. Enlist the types of synergism. 2M
10. Enlist the factors affecting dose of a drug. 2M
11. Write the formulas used to calculate dose of a drug according to body weight. 2M
12. Write the formulas used to calculate dose of a drug according to age. 2M

TINA RAJU, ASSOCIATE PROFESSOR, DR. MOOPEN’S COP, WAYANAD