Body and drug basics
Half-life
Last updated
Half-life is the time it takes for half of the substance present in the body to be gone. If something has a half-life of one hour, roughly half remains after an hour, a quarter after two hours, an eighth after three. It measures how quickly something disappears, and it holds regardless of how much was there to begin with, which is why it can be quoted as a single number.
Why it matters here
Half-life is what people mean by short-acting and long-acting, and it sets how quickly levels rise and fall between one exposure and the next. A very short half-life also means a substance can be almost entirely gone before anything slow-moving in the body has had a chance to respond to it.
What it is not
Half-life is not how long an effect lasts. A substance can be cleared while the change it started carries on for much longer, and it can equally linger with nothing measurable happening. Quoted figures also vary between people, between species and between measurement methods, so a single number is an estimate rather than a constant.
Also called
- half-life
- half life
- halflife
- t1/2
- t½
- elimination half-life
- biological half-life
Written as t1/2 or t½, where t stands for time. Elimination half-life names the phase being measured, since levels can fall in more than one stage.
Compounds carrying this term
None on file yet. This term is in the controlled vocabulary but no compound record currently uses it — which is a statement about our coverage, not about the term.