Peptide Half-Life: Why the Schedule Follows From the Number

Half-life is the time it takes for half of a compound to be cleared. It sounds like a technical footnote and it decides almost everything practical: how often something is given, how long until levels stabilise, and how quickly they fall once you stop.

It is also the number most often quoted inconsistently, because the same compound name can cover molecules with very different clearance.

A comparison across commonly discussed compounds

What half-life determines

Dosing frequency

A compound cleared in twenty minutes cannot maintain a level with weekly administration. Short half-life means frequent dosing or accepting brief pulses — which for GHRH analogues is the intent rather than a limitation.

Time to steady state

Roughly four to five half-lives are needed to reach a stable level with repeated dosing. For a weekly drug with a seven-day half-life, that is about a month — which is why changes to incretin drugs take weeks to fully express, and why judging one after a few days measures noise.

Washout

The same arithmetic runs in reverse. Four to five half-lives after stopping, a compound is largely cleared — days for a short one, a month or more for a long one.

Why quoted figures disagree

1. Different molecules, same name. The CJC-1295 case is the clearest, and TB-500 versus thymosin beta-4 is another.

2. Different species. Rodent clearance is generally faster; a figure from a mouse study does not transfer.

3. Different routes. Subcutaneous, intravenous and oral produce different curves for the same molecule.

4. Free versus bound. For compounds like IGF-1 that circulate bound to carrier proteins, the two figures differ enormously. See IGF-1 LR3.

5. Not measured in humans. For most research peptides, no human pharmacokinetic study exists, and quoted numbers are estimates.

The practical consequences

• Short half-life compounds are timing-sensitive. When you administer them matters, because the window is narrow.

• Long half-life compounds are patience-sensitive. Nothing about a change is fully visible for weeks.

• Receptor desensitisation interacts with this. Continuous exposure can invert an effect entirely — see gonadorelin, where pulsatile stimulates and continuous suppresses.

Frequently asked questions

How long until a weekly drug reaches steady state?

Roughly four to five half-lives — about a month for a compound with a seven-day half-life.

Why do sources disagree on CJC-1295?

Because the DAC and no-DAC forms are different molecules. One is measured in days, the other in minutes, and both can be quoted correctly.

Does half-life tell you how long an effect lasts?

Not directly. It describes clearance of the compound. Downstream effects can outlast its presence, and receptor-level changes can persist longer still.