Peptide Half-Life Explained: Clearance, Steady State and Dosing Frequency
A half-life is the time it takes for the amount of a substance in the body to fall by half. It is the single most useful number for understanding how long a peptide lasts, how often it is dosed, and how levels build up over repeated doses.
What half-life means
Most peptides are cleared by first-order elimination: a constant fraction is removed per unit of time, so the amount drops by half every half-life no matter how much you start with. After each half-life, the remaining amount halves again:
| Half-lives elapsed | Remaining |
|---|---|
| 1 | 50% |
| 2 | 25% |
| 3 | 12.5% |
| 4 | 6.25% |
| 5 | 3.13% |
| 7 | 0.78% |
| 10 | 0.1% |
That is why "five half-lives" is the usual rule of thumb for a substance to be essentially gone: about 97% has been eliminated.
The underlying formula is C(t) = C₀ × e−kt, where the elimination rate constant k = ln 2 / t½. The Kinetic calculator adds up this curve for every dose you log.
Apparent half-life after an injection
For subcutaneous injections, the peptide has to be absorbed from under the skin before it can be eliminated. When absorption is slower than elimination, the observed decline reflects the absorption rate instead. Labels often report this apparent half-life, which can be much longer than the half-life of the same molecule given intravenously. Kinetic lists the half-life relevant to the usual route for each compound.
Steady state and accumulation
With repeated doses at a fixed interval, each new dose adds to what is left of the previous ones. Levels climb until the amount eliminated between doses equals the amount given; this plateau is steady state, reached after roughly 4 to 5 half-lives regardless of the dose.
How much levels build up depends on the dosing interval (τ) relative to the half-life. The accumulation ratio is 1 / (1 − 2−τ/t½), and the peak-to-trough swing within each interval is 2τ/t½:
| Dosing interval | Steady-state peak vs first dose | Peak-to-trough ratio |
|---|---|---|
| Every 0.25 half-lives | 6.29x | 1.19x |
| Every 0.5 half-lives | 3.41x | 1.41x |
| Every half-life | 2x | 2x |
| Every 2 half-lives | 1.33x | 4x |
| Every 4 half-lives | 1.07x | 16x |
Dosing more often than once per half-life produces large accumulation and smooth levels. Dosing much less often produces little accumulation but big swings between peaks and troughs.
Two examples
- Semaglutide has a half-life of about 6.9 days and is given weekly, so the interval is about one half-life. Levels at steady state are roughly 2 times those after the first dose and are reached after about 4 to 5 weeks.
- BPC-157 has an estimated half-life of about 30 min. With once-daily administration, each dose is essentially cleared long before the next, so there is no meaningful accumulation.
Limits of a simple model
Real pharmacokinetics vary with body weight, kidney and liver function, injection site, formulation and dose. Many research peptides have little or no human data, so their half-lives are estimates. Treat any curve, including ours, as an approximation rather than a measurement.
FAQ
How many half-lives until a peptide is out of your system?
About five half-lives, after which roughly 97% of a single dose has been eliminated.
How long does it take to reach steady state?
About 4 to 5 half-lives of consistent dosing, regardless of the dose size. A longer dosing interval relative to the half-life means less accumulation but larger peaks and troughs.
Why is the half-life after a subcutaneous injection longer?
Absorption from under the skin can be slower than elimination, so the measured decline reflects absorption. This is called an apparent half-life.