Elimination Half-Life

The elimination half-life (often written t-half) is the time required for the concentration of a medicine in the body to decrease by 50%. It is one of the most useful pharmacokinetic parameters because it determines how often a medicine must be dosed to maintain therapeutic concentrations, how long it takes to reach steady state, and how long the medicine will linger after discontinuation.

A practical rule of thumb is that steady-state concentrations are reached after approximately 4-5 half-lives of consistent dosing, and that 4-5 half-lives are needed for the medicine to be essentially eliminated from the body after stopping. For a medicine with a half-life of 24 hours, this means roughly 4-5 days to reach steady state and 4-5 days for washout. For a medicine with a half-life of 1 hour, this means hours rather than days. Medicines with very long half-lives (amiodarone, with a half-life of weeks to months; fluoxetine and its active metabolite norfluoxetine, with combined half-lives of 4-16 days) require particular attention because effects persist long after the medicine is stopped.

Half-life depends on two factors: clearance (the rate at which the body removes the medicine through metabolism and excretion) and volume of distribution (how widely the medicine spreads through body tissues). A medicine that distributes widely into tissues will have a longer half-life even if cleared rapidly. Patient factors that prolong half-life include reduced kidney function (for renally cleared medicines), reduced liver function (for hepatically metabolized medicines), advanced age, congestive heart failure, and CYP450 enzyme inhibition.

Clinically, half-life guides dosing frequency. A medicine with a short half-life given once daily will produce wide peak-trough swings; given multiple times daily, it produces steadier levels. Conversely, medicines with long half-lives can usually be given once daily with stable levels. The half-life also informs how quickly a missed dose matters: missing a dose of a long-half-life medicine (warfarin, t-half about 36 hours; amlodipine, t-half about 36 hours) produces little immediate change, whereas missing a dose of a short-half-life medicine (immediate-release metoprolol, t-half about 3-7 hours) can result in noticeable loss of effect.

A common misconception is that half-life predicts duration of action. The two are related but not identical. Some medicines have a longer duration of action than their half-life would suggest because of active metabolites (fluoxetine), tight receptor binding (proton pump inhibitors irreversibly inhibit the pump), or downstream effects that persist after the medicine is gone. Conversely, medicines may have a shorter clinical effect than their plasma half-life suggests if their target dissociates rapidly. Another misconception is that half-life is fixed. It can change with disease, age, and medicine interactions.

Prescribers use half-life to design dosing schedules, time medicine discontinuation around procedures or pregnancies, and predict the time course of medicine interactions.