A half-life is the time it takes for the amount of a compound in your body to fall by half. That single number quietly decides almost everything about a protocol — how often a dose is given, how long it lingers after the last one, and how repeated doses stack up over time.
In plain terms: it is a compound's "fade rate."
The halving, step by step
If a compound's half-life is 24 hours, then starting from a full dose:
| Time | Remaining |
|---|---|
| 0 h | 100% |
| 24 h | 50% |
| 48 h | 25% |
| 72 h | 12.5% |
| 96 h | 6.25% |
| 120 h | ~3% |
Notice the pattern: each step removes half of what is left, not a fixed amount. Early on the drop is large (50% of the whole dose in one half-life); later it is tiny (a few percent). Pharmacologists call this first-order elimination — the body clears a constant *fraction* per unit time, not a constant *quantity*1.
In plain terms: the compound fades fast at first, then more and more slowly, like a fading echo.
The five-half-lives rule
After five half-lives only about 3% of a single dose remains (0.5 to the power of 5 is roughly 0.03), so the compound is treated as substantially cleared1. That is the basis of the widely used "five half-lives to clear" rule.
The same timescale, run in reverse, is how long *repeated* doses take to build up to a plateau — covered in steady-state and accumulation.
The math
`` remaining fraction = 0.5 ^ (time ÷ half-life) ``
Every half-life page on this site plots exactly this curve for a single dose. When doses repeat before the last one clears, they stack — which is steady-state accumulation.
Why dosing cadence follows the half-life
A compound with a multi-day half-life is still substantially present when the next dose is due, so it can be dosed infrequently. A compound with a half-life of hours is nearly gone quickly, so research protocols using it dose more often.
| Compound | Reported half-life | Typical cadence |
|---|---|---|
| Native GLP-1 hormone | ~minutes | (not a drug) |
| Tirzepatide | ~5 days3 | Weekly |
| Semaglutide | ~7 days2 | Weekly |
In plain terms: if a compound is still half-present a week later, a weekly dose keeps levels in a workable range. This is exactly why GLP-1 medications are weekly while short peptides appear on daily or twice-daily research schedules.
What a half-life is not
- Not the same as how long it works. Half-life tracks the blood *level*, not the *effect*. Some compounds keep acting after they have cleared — the BPC-157 half-life page unpacks that gap.
- Not a personal number. The published figure is an average; individual clearance varies with route, formulation, and physiology.
Half-life is a population estimate. The curves on this site are models, not measurements — an honest picture of blood level, not a personal clearance timeline.