Peptide Half-Life: Why Schedules Differ
Why some peptides are injected daily and others weekly — DPP-4 degradation, albumin binding, and what half-life means for steady state and a missed dose.
Some peptides are administered once a week. Others would be useless at anything less than daily. The difference is almost entirely one number: half-life.
Understanding it explains why dosing schedules look the way they do, why protocols escalate gradually, and why a delayed dose means something different for one compound than for another.
What does half-life actually mean?
Half-life is the time it takes for the concentration of a substance in blood plasma to fall by half. It is not the duration of effect, and it is not how long the substance is detectable — but it governs both.
Two consequences follow from it, and both are practical:
- Reaching steady state takes about four to five half-lives. Until then, each dose is added to a residue that has not yet stabilised, and the average concentration is still climbing.
- Clearing the substance takes about the same. Stopping is not instantaneous for a long-half-life compound.
Why do native peptides disappear so fast?
The body is built to take peptides apart. They are, structurally, exactly what digestive and plasma proteases exist to cleave.
Native GLP-1 is the standard example. The enzyme DPP-4 (dipeptidyl peptidase-4) cleaves it at the N-terminus, and Deacon and colleagues showed in 1995 that the hormone is degraded within roughly two minutes of reaching the circulation, whether given subcutaneously or intravenously.
Two minutes is not a dosing schedule. It is why the hormone itself was never a drug, and why an entire class of medicines exists to engineer around that fact.
The three ways to extend it
1. Block the cleavage site
If DPP-4 cuts at a specific position, put something there it cannot cut. Semaglutide places Aib — an unnatural amino acid — at position 8, exactly where the enzyme attacks.
2. Bind to albumin
Albumin is the most abundant protein in plasma and is cleared slowly. Attaching a fatty acid side chain that binds it reversibly turns albumin into a reservoir: it holds most of the circulating peptide and releases it gradually. This is the mechanism behind both semaglutide and tirzepatide.
3. Make the molecule bigger
Small molecules are filtered out by the kidney. Increasing effective size — through PEGylation or fusion to a larger protein — slows renal clearance. This approach is used more in protein therapeutics than in the peptides discussed here.
Which peptides have a published half-life?
Fewer than you would expect from reading dosing advice online. Here is every compound covered on this site, with the figure where one has actually been published and the reason where one has not.
| Compound | Plasma half-life | What that rests on |
|---|---|---|
| Native GLP-1 | ~2 minutes | Measured in humans; DPP-4 cleaves it at the N-terminus within minutes of release (Deacon, 1995) |
| Semaglutide | ~1 week | Aib substitution plus albumin binding; stated in the FDA prescribing information |
| Tirzepatide | ~5 days | Fatty-acid side chain binding albumin; stated in the FDA prescribing information |
| Semax | No figure citable here | The Pro-Gly-Pro tail was added specifically to slow peptidase cleavage, so it outlasts the bare fragment — but the pharmacokinetic work sits in Russian-language literature |
| BPC-157 | No published human figure | No characterised human pharmacokinetic profile exists in the public literature |
| TB-500 | No published human figure | Published work on the fragment is analytical and doping-control chemistry, not pharmacokinetics |
| Epitalon | No published human figure | The evidence base is cell-culture and animal work, largely from one research programme |
| KPV | No published human figure | Preclinical only, and the best-studied route is targeted delivery to the gut rather than systemic exposure |
| MOTS-c | No published human figure | Human data are observational measurements of endogenous levels, not administration studies |
That split lines up almost exactly with which compounds are actually approved, which is not a coincidence.
Half of that table is empty, and the emptiness is the finding rather than a gap in our reading of the research. A published half-life is a by-product of formal pharmacokinetic study, which is in turn a by-product of drug development. The compounds with numbers are the ones that went through it.
Which has a direct consequence: for the compounds without a figure, any dosing interval circulating online was not derived from a measured clearance rate, because no measured clearance rate has been published. It was derived from convention, from animal data, or from nothing in particular.
What this means for a schedule
Escalation is pharmacology, not caution theatre. If steady state arrives after four or five half-lives, then for a weekly compound the concentration is still rising well over a month in. Protocols that step up gradually are tracking that curve.
A delay is not equally forgiving. With a half-life of a week, a dose taken a day late lands on a plasma level that has barely moved. With a half-life of hours, the same delay can mean the concentration has effectively reached zero and the profile restarts.
The interval is part of what was administered. A weekly compound taken every ten days is a different exposure from the same amount taken every seven — the quantity per injection is identical, the average concentration is not.
Half-life figures are population averages from pharmacokinetic studies. Individual clearance varies with kidney and liver function, body weight and other medications. Nothing here is a schedule recommendation — the authority for any specific product is its prescribing information and the prescriber.
Why this makes tracking worth doing
If the interval is part of the exposure, then a schedule remembered approximately is data lost. What is worth having is the actual date and time of each administration, and the gaps as they really were rather than as they were intended.
The arithmetic that turns a vial into what actually goes into the syringe is a separate problem, and the reconstitution calculator handles it — but the interval is the half of the exposure that no calculator can reconstruct after the fact.
PeptCycle is built around that interval rather than around the dose. It schedules forward from the frequency you set, fires the reminder at the time you chose, and records the gap that actually happened next to the one you planned. It stays on the phone; there is no account to make.
Frequently asked questions
Which peptides have a published half-life?
Only the ones that went through formal drug development. Native GLP-1 is about two minutes, tirzepatide about five days and semaglutide about a week, all measured and documented. For BPC-157, TB-500, Epitalon, KPV and MOTS-c no human pharmacokinetic figure has been published at all — which means any dosing interval quoted for them was not derived from a measured clearance rate.
What does half-life mean?
The time it takes for the concentration of a substance in blood plasma to fall by half. It is the single number that most determines how often something has to be administered.
Why is native GLP-1 useless as a medicine?
Because the enzyme DPP-4 cleaves it at the N-terminus within about two minutes of release. Deacon and colleagues demonstrated this in humans in 1995. Every GLP-1 medicine is an attempt to engineer around that degradation.
How long does it take to reach steady state?
As a general pharmacokinetic rule, roughly four to five half-lives. For a compound with a half-life of about a week that means several weeks — which is why trial protocols escalate gradually instead of starting at the target dose.
Does a missed dose matter more for a short or a long half-life?
More for a short one. With a long half-life a substantial fraction is still circulating a day later, so a delay changes the curve modestly. With a short half-life, the same delay can mean the concentration has effectively reached zero.
Sources
- Deacon C. et al., Both subcutaneously and intravenously administered GLP-1 are rapidly degraded from the NH2-terminus, Diabetes 1995
- Lau J. et al., Discovery of the Once-Weekly GLP-1 Analogue Semaglutide, J Med Chem 2015
- FDA — Mounjaro (tirzepatide) prescribing information
- FDA — Ozempic (semaglutide) prescribing information