How Long a Peptide Lasts

The short version

A peptide is a short chain of amino acids. The body treats one much the way it treats food. Enzymes cut it apart, and the kidneys filter out what is left. Most peptides vanish from the blood within minutes.

Half-life is the word for how fast that happens. It is the time it takes for half of what is in the blood to disappear. A short half-life means the molecule leaves quickly. A long one means it lingers.

Chemists change the molecule to slow the exit. They swap in amino acids the enzymes cannot cut. They hang a fatty chain on it so it grabs a blood protein and rides along. Small changes, large effect: minutes become days.

This desk reads four peptides through that single lens. It reports measured numbers, and it names the animal each number came from, because a figure from a rat is not a figure from a person. It does not turn a number into a schedule.

What a half-life is, and what it is not

An elimination half-life is a measurement. It describes how fast a molecule disappears from plasma in a stated population, by a stated route, at a stated dose. Change any of those three and the number changes with it.

A dosing interval is something else entirely. It is a clinical judgement made against an indication, a population and a monitoring plan. The two are related. They are not the same object, and one does not follow from the other by arithmetic.

The compounds on this desk make the gap unusually visible. In healthy adults, single subcutaneous doses of CJC-1295 gave an estimated half-life of 5.8 to 8.1 days — while mean plasma growth hormone stayed elevated for six days or more and IGF-1 for nine to eleven days, remaining above baseline as long as 28 days after repeated dosing ^[4]. In 13 healthy men, two weeks of tesamorelin raised IGF-1 by 181 micrograms per litre ^[21] on a molecule whose own plasma residence is short. In healthy male volunteers, ipamorelin cleared with a terminal half-life of roughly 2 hours, yet the growth-hormone response it triggered arrived as a single discrete pulse peaking about 40 minutes after the infusion ^[11].

Three different relationships between exposure and effect, in three molecules from two receptor families. That is the reason this desk reports the measurement and declines the schedule.

Why these peptides are injected rather than swallowed

The obstacle is the digestive tract itself. Stomach acid and intestinal proteases exist to break peptide bonds, and the gut wall is not built to pass molecules of this size intact. A peptide taken by mouth is largely destroyed before absorption and poorly absorbed after it.

Every human study cited on this site used a route that bypasses the gut. CJC-1295 was given subcutaneously ^[4][5]. Ipamorelin was given as intravenous infusions ^[11]. The semaglutide outcome trials used weekly subcutaneous injection ^[14][15][16]. The tesamorelin trials used daily subcutaneous injection ^[20][21][22].

One exception exists in this set. Semaglutide has an oral formulation, co-formulated with an absorption enhancer and requiring strict fasted administration, because absorption across the gut is the limiting step rather than an afterthought. The specific bioavailability figures for that formulation sit outside this digest's reference set, so they are not quoted here.

Protease resistance and gut permeability are separate problems. Solving the first buys circulating time. It does not, by itself, make a peptide orally active.

Four molecules, four ways of buying time

CJC-1295 carries four amino-acid substitutions that block enzymatic cleavage, and in its DAC form a linker that forms a covalent bond with circulating serum albumin. In rats, that conjugate produced a four-fold increase in growth-hormone exposure over two hours compared with unconjugated hGRF(1-29), and the peptide remained detectable in plasma beyond 72 hours ^[7].

Ipamorelin is the useful contrast. Its non-standard residues resist proteases, but nothing anchors it to a carrier protein. It clears at 0.078 litres per hour per kilogram in healthy male volunteers, with a steady-state volume of distribution of 0.22 litres per kilogram ^[11]. Resistance without an anchor buys hours.

Semaglutide uses acylation. A C18 fatty di-acid attached through a spacer binds albumin strongly but reversibly, and a substitution at position 8 blocks the enzyme that dismantles native GLP-1. The molecule the trials administered was given once weekly ^[14][15][16].

Tesamorelin takes the minimal route: a single acyl group at the N-terminus, enough to defeat the same enzyme, with no carrier binding. Its plasma exposure is correspondingly brief, and its measured effects on the growth-hormone axis outlast it ^[21][22].

What this desk is, and what the name is not

There is no institute. No faculty, no laboratory, no research programme, no accreditation, no affiliation with any university, hospital, manufacturer or clinic. The domain names the subject, not a status. Nothing here is peer-reviewed, and no original research is conducted.

What this is: one editorial desk reading 22 published sources about four compounds, and writing down what they measured.

The citation rule is strict and worth stating plainly. Every quantitative claim on this site carries a bracketed number pointing to the reference list. Where a figure exists in the wider literature but not in these 22 sources, the page says so and leaves the number out rather than attaching it to a citation that does not support it. Half-life figures are the most commonly mis-sourced numbers in this subject, which is exactly why the rule is applied hardest to them.