Half-life

In pharmacology, the elimination half-life is the time needed for the concentration of a drug in blood plasma to fall to 50 percent of its starting value. If a drug has a half-life of four hours, a level of 100 units drops to 50 after four hours, 25 after eight and 12.5 after twelve. The term comes from physics, where it describes radioactive decay.

For most drugs at ordinary doses, the body removes a constant fraction per unit of time, known as first-order elimination, so the half-life is the same whatever the dose. It depends on how quickly the liver and kidneys clear the drug and on how widely the drug spreads into tissues: slower clearance or a larger volume of distribution means a longer half-life. After about five half-lives roughly 97 percent of a dose is gone, and with regular dosing the level settles at a steady state after a similar number of half-lives.

Half-lives range from seconds for adenosine to weeks or months for amiodarone. Drugs with short half-lives must be taken more often, and a half-life that lengthens in older people or in kidney or liver disease can let a drug build up to toxic levels.