The word posology is derived from the Greek words ‘posos’ meaning how much and ‘logos’ meaning science. 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.

Factors Affecting Posology

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


The pharmacokinetics of many drugs changes with age. So while determining the dose of a drug, the age of an individual is of great significance. Children and old people need a lesser amount of drugs than the normal adult dose because they are unable to excrete drugs to that extent as adults. Children can tolerate relatively larger amounts of belladonna, digitalis and ethanol, whereas elderly patients are more sensitive to some drug effects. For example, hypnotics and tranquillizers may produce confusion states in them.


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. 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, narcotic and non-narcotic analgesics etc.


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. However, the dose expressed in this fashion may not apply in cases of obese patients, children and malnourished patients. It should be calculated according to body weight.

Route of administration:

Intravenous doses of drugs are usually smaller than oral doses because the drugs administered intravenously enter the bloodstream directly. Due to this reason, the onset of drug action is quick with the intravenous route and this might enhance the chances of drug toxicity. The effectiveness of drug formulation is generally controlled by the route of administration.

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. So the amount of drug which is very effective when taken before a meal may not be that much effective when taken during or after meals. The irritating drugs are better tolerated if administered after meals, for example, iron, arsenic and cod-liver oil should always be given after meals.

Environmental factors:

Daylight is a stimulant, enhancing the effect of stimulating drugs and diminishing the effect of hypnotics. Darkness is sedative. Hypnotics are more effective at night. The amount of barbiturate 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.

Emotional factors:

The personality and behaviour of a physician may influence the effect of the drug especially the drugs which are intended for use in a psychosomatic disorder. Females are more emotional than males and require less dose of certain drugs.

Presence of disease:

Drugs like barbiturates may produce an unusually prolonged effect in patients having liver cirrhosis. Streptomycin is excreted mainly by the kidney may prove toxic if the kidney of the patient is not working properly.


The drugs which are slowly excreted may be built up a sufficiently high concentration in the body and produce toxic symptoms if it is repeatedly administered for a long time e.g. digitalis, emetine and heavy metals. This occurs due to the accumulative effect of the drug.

Additive effect:

When the total pharmacological action of two or more drugs administered together is equivalent to the sum of their pharmacological action, the phenomenon is called an additive effect. For example, the combination of ephedrine and aminophylline in the treatment of bronchial asthma.


When two or more drugs are used in the combination their action is increased. The phenomenon is called synergism.


When the action of one drug is opposed by the other drug on the same physiological system is known as drug antagonism. The use of antagonistic responses to drugs is valuable in the treatment of poisoning e.g. milk of magnesia is given in acid poisoning where the alkaline effect of milk of magnesia neutralises the effect of acid poisoning.


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 haemorrhage and a small dose of quinine may produce ringing in the ears.


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., smokers can tolerate nicotine, and alcoholics can tolerate a large quantity of alcohol.

Metabolic disturbances:

Changes 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 a rise in body temperature. They have no antipyretic effect if the body temperature is normal.

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