half-life
The half-life is the time it takes for the amount of drug in the plasma to drop by half. If you start at 100 units and the half-life is 4 hours, you will have about 50 after 4 hours, 25 after 8, and so on — each interval halving what remains. It is one of the most quoted numbers in pharmacology because it tells you, at a glance, how fast a drug fades.
Half-life is a derived quantity, not a fundamental one: it depends on both how fast the body clears the drug and how widely the drug is distributed, through the relation t½ ≈ 0.693 × volume of distribution / clearance. A large volume of distribution lengthens half-life (the drug hides in tissues), while high clearance shortens it.
Two practical rules of thumb follow. After about 4–5 half-lives a drug is roughly 94–97% eliminated (practically gone), and the same 4–5 half-lives are needed to reach steady state during repeated dosing. The neat halving behaviour holds only for first-order kinetics; under zero-order (saturated) kinetics there is no single constant half-life.
Aspirin's active form has a short half-life of a few hours, while amiodarone's is weeks to months — which is why amiodarone takes a very long time both to build up and to wash out after stopping.
Long half-life means slow build-up and slow washout.
Half-life is governed by clearance and volume of distribution — not the other way round. Changing the dose does not change the half-life in first-order kinetics.