Peptide half-life, explained

Elimination is proportional: the body removes a fixed fraction of what is present per unit time, not a fixed amount. That is why the interval to halve is constant. Starting from any amount, half is gone after one half-life, a quarter remains after two, an eighth after three.

The practical consequence is that the tail is long but the bulk clears fast. After three half-lives you are down to about an eighth; after five, roughly 3% remains, which is the conventional threshold for calling something cleared. Five half-lives is the number worth remembering, and it does double duty — it is also how long repeated dosing takes to reach its plateau.

A half-life on its own predicts nothing about repeated dosing. What matters is the half-life relative to the dosing interval.

If the interval is much longer than the half-life, each dose has essentially cleared before the next arrives. Levels are a sawtooth: sharp rise, complete fall, repeat. Nothing accumulates, and every dose behaves like the first one.

If the interval is shorter than or similar to the half-life, leftovers from previous doses are still present when the next lands. Those leftovers stack, the baseline creeps upward, and levels keep climbing until the amount cleared per interval equals the amount going in. That plateau is steady state, and it arrives after about five half-lives. A weekly compound with a week-long half-life will sit meaningfully higher at week six than it did at week one, with nothing about the routine having changed.

Published half-lives are population averages from pharmacokinetic studies, usually a mean across a modest number of participants. Individual values scatter around that mean for entirely ordinary reasons: kidney and liver function, body composition, age, concurrent medications, and — for subcutaneous injection — how well perfused the injection site happened to be.

For most well-studied compounds the reported figure is reliable enough to reason with. For compounds without human pharmacokinetic studies, the number circulating online may be extrapolated from animal work, inferred from a structural analogue, or simply repeated without a source. Treat those as order-of-magnitude estimates rather than measurements.

The most common misreading is treating half-life as duration of effect. They are different quantities and often diverge sharply.

A compound can act through a receptor it binds tightly and dissociates from slowly, so the effect outlasts measurable levels. It can trigger a downstream cascade that continues after the trigger is gone. It can produce a structural change — tissue remodelling, altered gene expression — that persists indefinitely. Conversely, tolerance can blunt an effect while concentrations remain high.

Half-life describes the molecule's exit. Effect duration describes the biology it set in motion. Reasoning about one from the other is where most confident-sounding wrong conclusions come from.