16th May 2025

Half-life

Elimination half-life is the time required for the concentration of a particular substance or medication to decrease by half of its starting dose in the body. Different medications have different half-lives, but all medications follow the same following rule: after one half-life 50% of the initial drug amount is removed from the body. There are two commonly followed patterns for drug elimination – first order and zero order kinetics. First order kinetics occur when the rate of elimination is proportional to the amount of drug in the body i.e. the higher the concentration the greater the rate of elimination. Most medications follow first order kinetics. A few medications follow zero order kinetics, most notably phenytoin, ethanol, and salicylates (at high doses). Zero order kinetics occur when a constant amount of drug is eliminated per unit time.