half-life
(symbol: t½) n. in pharmacokinetics, the time necessary for the concentration in the blood of an administered drug to fall by 50%. Half-life is a function of the rate of clearance of a drug and its volume of distribution in various body systems; it is expressed by the equation t½ = (0.7 × volume of distribution)/clearance. Clinically, half-life varies among individuals as a result of age, disease states, or concurrent administration of other drugs. Half-life is useful in predicting the duration of effect of a drug and the time required for a drug to reach a state of equilibrium (steady state) in the body—that is, when the amount of drug administered is equal to that excreted. Generally, steady state is predicted to be achieved after 4 to 5 half-lives of a drug; for example, if a drug has a measured half-life of 8 hours (and its dosing schedule remains the same), steady state would
be anticipated within 32 to 40 hours.