Learn pharmacokinetics
Half-life: how long a drug stays in your body
Medically reviewed by Javeria Muneer, Pharmacist · Last reviewed:
The elimination half-life (t½) is the time it takes for the concentration of a drug in plasma to fall by half once absorption and distribution are complete. If a drug has a half-life of 4 hours, a level of 20 mg/L becomes 10 mg/L after 4 hours, 5 mg/L after 8 hours and 2.5 mg/L after 12 hours.
Half-life is not a property you can change with one tablet. It comes from two underlying parameters: clearance (CL), the volume of plasma cleared of drug per hour, and volume of distribution (V), how widely the drug spreads into tissues.
The formula
For a one-compartment model the elimination rate constant is k = CL / V, and the half-life is t½ = ln 2 / k, about 0.693 × V / CL. A drug with a large volume, such as one that dissolves into fat, has a long half-life even if the liver clears it efficiently. A drug with a small volume and fast kidney clearance, such as gentamicin in a healthy adult, has a short one.
Why it matters in practice
Half-life sets how often a medicine needs to be taken. Drugs with half-lives of a few hours usually need dosing two to four times a day; drugs with half-lives of a day or more can be taken once daily.
It also tells you how long effects last after stopping. After about five half-lives roughly 97% of the drug has gone. That is why a drug with a 36-hour half-life can still cause interactions a week after the last dose.
Kidney or liver disease lowers clearance, which lengthens half-life and causes accumulation. Dose adjustments in renal impairment exist for exactly this reason.
See it in the simulator
The mini simulator below shows a single IV bolus. Halve the clearance and watch the curve fall twice as slowly.
