It is the time that is required for concentration of a drug so that it falls to half of its initial concentration after reaching its peak level. The duration of action of a drug is known as its half life. This is the period of time required for the concentration or amount of drug in the body to be reduced by one-half. We usually consider the half life of a drug in relation to the amount of the drug in plasma.
Onset, duration and intensity of effect of drug depend on the rate of absorption, distribution and elimination through excretion. In case of oral route, plasma concentration gradually rises and reaches its peak value; then, it begins to fall down. Drug requires a minimum effective plasma concentration for the effects to start appearing. Once the level falls below the minimum effective plasma concentration, the effect is over.For example, after intravenous administration, if maximum concentration is 16 mg and the half life is 2 hours, after 2 hours 8 mg will be left, and so on.