The response to a drug varies from one individual to the another. The following are the factors that are responsible for variation in drug effects.
AGE:-
Children are hyperreactive to certain drugs. The reasons are the immaturity of renal functions or the poor development of enzymes needed for inactivation. So a lesser dose must be given for children than for adults.
Young’s formula = Age ×adultdose/Age+12
- BODY WEIGHT:- Body weight has a definite influence on the contraction of the drug at the site of action. So the dose of the drug must be suitably adjusted in the case of obese individuals.
SEX:- Women are more susceptible to the effect of certain drugs eg:- morphine produces more excitation in women than in men.
ROUTE OF ADMINISTRATION:- The route of absorption of a drug differs from the route of administration. The dose also varies with the route of administration. Eg:- intravenous dose of a drug is less than subcutaneous.
TIME OF ADMINISTRATION:- This factor has a definite effect on drug absorption and hence on its effect. Eg:- Drugs that produce nausea and vomiting (due to gastric irritation) should be taken after food. But anthelmintics should be taken on an empty stomach.
PHYSIOLOGICAL FACTORS:- Body temperature and acid-base status are some factors which modify drug effects . Eg:- Salicylates lower body temperature only in fever but not in normal individuals.
PATHOLOGICAL STATE:- The effect of a drug may be modified in pathological conditions. Eg:- Hyperthyroid.
GENETIC FACTORS:- The effects of a drug may vary due to genetic factors like inherited enzyme deficiencies.