Skip to content
Peptide Half-LifePowered by Peptide Library

Interpreting the numbers

Half-life vs duration of action

Why a peptide’s half-life is not the same as how long its effect lasts: receptor binding, downstream signalling, active metabolites and why “five half-lives” is arithmetic, not a washout guarantee.

Last reviewed 2026-11-08 · Sources cited for every value

AnswerHalf-life is a pharmacokinetic measure: how fast concentrations fall. Duration of action is pharmacodynamic: how long an effect lasts. They can move together, but they are different quantities, and one cannot be read off the other.

Why they come apart

Receptor binding and signalling

An effect depends on what happens at and after the receptor: binding kinetics, signalling cascades and changes in gene expression can persist or fade on their own clocks.

Thresholds and saturation

If an effect is already maximal, halving the concentration may change little. If it needs a minimum concentration, it can stop while compound remains.

Active metabolites

Breakdown products may be active (or measured by an assay) and follow their own half-lives.

Tissue exposure

Plasma is not the site of action. Tissue concentrations can lag behind or outlast plasma levels.

About “five half-lives”

In the idealized model, after n half-lives the fraction remaining is 0.5n, so five half-lives leaves 3.125 %. That is why pharmacology texts use it as a rough landmark for when a compound is “mostly” eliminated, or when repeated administration approaches steady state.

It is not a guarantee of anything. Real decline can be multi-phasic, metabolites can persist, and individual factors change clearance. This site never presents five half-lives as a washout period or a safety threshold.

See the curve on the half-life calculator, or read how half-life works.

Frequently asked questions

Can an effect last longer than the half-life?

Yes. A compound can trigger signalling, gene expression or hormone release that continues after concentrations fall. A short-lived growth hormone secretagogue, for example, causes a hormone pulse whose downstream effects outlast the peptide.

Can an effect end before concentrations fall?

Yes. Receptors can desensitize, or the effect may need concentrations above a threshold, so it fades while some compound is still present.

Is five half-lives a safe washout?

No. Five half-lives leaves about 3 % in the idealized model, a useful mathematical landmark. It is not a safety threshold, ignores metabolites and individual variation, and says nothing about when effects end.