Posology, Factors Influencing Dose, Calculation of Doses

Posology, Factors Influencing Dose, Calculation of Doses
January 23, 2019 0 Comments



The word Posology is derived from the Greek words posos meaning how much and logos meaning science.  POSOLOGY, FACTORS INFLUENCING DOSE, CALCULATION OF DOSES So posology is a branch of medical science which deals with the dose or quantity of drugs that can be administered to a patient to get the desired pharmacological actions.

The dose of a drug cannot be fixed rigidly because there are so many factors that influence the doses. these factors are age, condition of the patient, severity of the disease, tolerances both natural and acquired, idiosyncrasy, route of administration, the formulation used, drug interactions, and rate of eliminations.


The dose range is usually based on the average requirements of an adult patient.

The following are some of the factors which influence the dose-

  • AGE– the pharmacokinetics of many drugs changes with age.

Children and old people need a lesser amount of drugs than the normal adult dose.

Children can tolerate relatively larger amounts of belladonna, digitalis, and ethanol

e,g. hypnotics and tranquilizers which may produce confusion state in them.

2) SEX women do not always respond to the action of drugs in the same manner as it is done in men.

Morphine and barbiturates may produce more excitement before sedation in women.

Special care should be taken when drugs are administered during menstruation, pregnancy, and lactation.

Strong purgatives such as aloe should be avoided during menstruation.

There are certain drugs that on the administration to the mother are capable of crossing the placenta and affecting the foetus. E,g. alcohol, barbiturates, narcotics and non-narcotic analgesic, etc.

During lactation, the drugs like antihistamine morphine and tetracycline which are excreted in milk should be avoided or given very cautiously to the mothers who are breastfeeding the babies.

3) BODY WEIGHTS– The average dose is mentioned either in terms of mg per kg body weight or as a total single dose for an adult weighing between 50-100 kg.

It should be calculated according to body weight.

4) ROUTE OF ADMINISTRATION– intravenous doses of drugs are usually smaller than the oral doses because the drugs administered intravenously enter the bloodstream directly.

Due to this reason, the onset of drug action is quick.

5) TIME OF ADMINISTRATION– the presence of food in the stomach delays the absorption of drugs.

The drugs are more rapidly absorbed from the empty stomach.

The irritating drugs are better tolerated if administered after meals, e,g. iron, arsenic, and cod-liver oil should always be given after meals.


6) ENVIRONMENTAL FACTORS– Daylights is a stimulant.

Hypnotics are more effective at night.

The amount of barbiturates required to produce sleep during the daytime is much higher than the dose required to produce sleep at night.

Alcohol is better tolerated in cold environments than in summer.

7) EMOTIONAL FACTORS– the personality and behavior of a physician may influence the effect of the drug especially the drugs which are intended for use in a psychosomatic disorder.

The females are more emotional than males and require less dose of certain drugs.

8) PRESENCE OF DISEASE– drugs like barbiturates and chlorpromazine may produce an unusually prolonged effect in patients having liver cirrhosis

During fever a patient can tolerate high doses of antipyretics than a normal person.

9) ACCUMULATION– the drugs which are slowly excreted may build up a sufficiently high concentration in the body and produce toxic symptoms.

Sometimes, a cumulative effect is desired in drugs like phenobarbitone in the treatment of epilepsy.

10) ADDITIVE EFFECT– when the total pharmacological action of two or more drugs administered together is equivalent to the sum of their individual pharmacological action, the phenomenon is called an additive effect.

11) SYNERGISM– when two or more drugs are used in the combination form, their action is increased.

e,g. Procaine and adrenaline combination increases the duration of action of procaine.

12) ANTAGONISM– When the action is opposed by the other drug on the same physiological system is known as drug antagonism.

e.g. milk of magnesia is given in acid poisoning.

13) IDIOSYNCRASY– An extraordinary response to a drug that is different from its characteristic pharmacological action is called idiosyncrasy.

The word idiosyncrasy has now been replaced by the term drug allergy.

For example, a small quantity of aspirin may cause gastric hemorrhage.

14) TOLERANCE– when an unusually large dose of a drug is required to elicit an effect ordinarily produced by the normal therapeutic dose of the drug, the phenomenon is termed drug tolerance.

e.g. smokes can tolerate nicotine, an alcoholic can tolerate a large quantity of alcohol.

The drug tolerance is of two types- 1) true tolerance 2) pseudo tolerance


15) TACHYPHYLAXIS– It has been observed that when certain drugs are administered repeatedly at short intervals, the cell receptors get blocked up and pharmacological response to that particular drug is decreased.

The decreased response cannot be reversed by increasing the dose.

This phenomenon is known as tachyphylaxis or acute tolerance.

16) METABOLIC DISTURBANCE– change in water-electrolyte balance and acid-base balance, body temperature, and other physiological factors may modify the effects of drugs.

Salicylates reduce body temperature only in case an individual has risen in body temperature.

They have no antipyretics effect if the body temperature is normal.

The absorption of iron from G.I.T is maximum if the individual has an iron deficiency anemia.




A.DOSES PROPORTIONATE TO AGE- there is a number of methods by which the dose for a child can be calculated from the adult dose.

1)YOUNGS FORMULA= age in years/ age in year+12 x  adult dose

The formula is used for calculating the doses for children under 12 years of age.


Dose for the child= age in year/ 20   x adult dose

The formula is used for calculating the doses for children between 4 to 20  years of age. This formula is considered better because it is easier and quicker to calculate the dose.


 Clark’s formula;

Dose for the child= child weights in kg/70    x   adult dose


The calculation of child dose according to surface area is more satisfactory and appropriate rather than the method based on age.

The method is more complicated than the method based on age.

The method is based on the following formula-

Percentage of adult  dose= surface area of child/ surface area of adult      x     100


  1. Age
  2. Sex
  3. Bodyweight and size
  4. Time of administration
  5. Route of administration
  6. Environmental condition
  7. Habit
  8. Rate of elimination
  9. The effect of drugs
  10. Purpose of medication
  11. Species
  12. The character of the drug.

Thank you…………………

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