half-life

The elimination half-life of a substance is the time it takes for its concentration in the blood — or relevant tissue — to fall by exactly half. It is one of the foundational numbers in pharmacokinetics, the branch of pharmacology concerned with how the body absorbs, distributes, metabolises, and excretes substances.

Half-life is not the same as how long effects are felt. A substance can stop producing noticeable effects well before it finishes clearing, or it may continue acting on receptors even as plasma concentrations drop. What half-life reliably predicts is elimination time: after roughly five half-lives, approximately 97% of a substance has left the body.

How it works · its role

The liver and kidneys are the primary organs of elimination. They metabolise and excrete substances at a roughly constant fractional rate — removing a fixed percentage per unit of time rather than a fixed amount. Because of this, the decline is exponential: concentration always halves in the same time interval, regardless of whether the starting level is high or low.

Some substances have a stable, predictable half-life that holds across doses. Others behave differently at higher concentrations, when metabolic enzymes become saturated and the rate of elimination slows. A few produce active metabolites with half-lives of their own, which effectively extend how long the substance influences the body even after the parent compound has cleared.

Relevance to substances & effects

Half-life shapes the practical experience of nearly every psychoactive substance. A short half-life — measured in minutes or a few hours — tends to produce a sharper, more defined arc with an abrupt end. A long half-life, measured in days, means the substance accumulates with repeated use; each dose adds to what the previous one left behind.

Among benzodiazepines, the contrast between short-acting and long-acting members illustrates this clearly: the same basic mechanism produces quite different experiences and risk profiles depending almost entirely on clearance speed. Fluoxetine's unusually long half-life — weeks, for some of its metabolites — is why it takes a month or more to fully wash out, and why it tends to produce milder discontinuation effects than shorter-acting antidepressants.

Long half-life opioids are used in substitution therapy partly because their slow, steady clearance smooths out the sharp peaks and troughs that drive craving and withdrawal. Conversely, substances with very short half-lives are more prone to producing rapid-onset withdrawal when use stops, because the body's exposure drops off steeply.