Half-Life & Pharmacokinetics
Pharmacokinetics (PK) is the study of what the body does to a drug — how it is absorbed, distributed, broken down, and cleared. For peptides, PK is the practical reason behind two of the most common questions people ask: why do I have to inject this? and why this dosing schedule? Understanding a few basics makes most dosing protocols stop looking arbitrary.
The four things PK tracks
Absorption (how it gets in), Distribution (where it goes), Metabolism (how it’s broken down), and Excretion (how it leaves) — often shortened to ADME.
What “half-life” means
A drug’s half-life is the time it takes for its concentration in the blood to fall by half. If a peptide has a 30-minute half-life, then 30 minutes after it peaks roughly half is gone, after another 30 minutes half of that remains, and so on.
Two practical takeaways:
- A drug is mostly cleared after about 4–5 half-lives. A 30-minute half-life means the peptide is largely gone within a couple of hours; a multi-day half-life means it lingers for weeks.
- Half-life is the main driver of how often you dose. Short half-life generally means frequent dosing (or accepting brief, pulsatile action); long half-life means infrequent dosing and steadier levels.
For approximate half-life values across the peptides on this wiki, see the Half-Life Reference Chart.
Why most peptides are injected, not swallowed
Most peptides have terrible oral bioavailability — very little of a swallowed dose survives to reach the bloodstream. There are two reasons, both covered in more depth in Peptide vs Protein:
- They get digested. Stomach acid and gut enzymes break peptides down the same way they break down dietary protein.
- They’re poorly absorbed. Even intact, most peptides are too large and water-loving to cross the gut wall efficiently.
So peptides are usually given by injection — most commonly subcutaneous (under the skin), sometimes intramuscular — which bypasses the gut entirely. Oral and other non-injected forms exist but require special engineering: oral Semaglutide uses an absorption enhancer, and MK-677 is orally active because it is a small molecule designed to mimic a peptide’s effect rather than a peptide itself.
Absorption and clearance
- Absorption. With a subcutaneous injection the peptide is absorbed gradually from the tissue into the blood, which blunts the peak and extends the time it stays active compared with an instant intravenous dose.
- Clearance. Peptides are cleared by being enzymatically broken down (peptidases in blood and tissue) and by kidney filtration. Small, unmodified peptides are cleared fast — hence the very short half-lives many of them have.
- Extending half-life. Drug designers deliberately slow clearance to make dosing more convenient. Common tricks include attaching the peptide to something large that the kidney can’t filter quickly, or making it bind to albumin (an abundant blood protein) so it rides along in circulation instead of being cleared. The “DAC” (Drug Affinity Complex) on CJC-1295 is exactly this kind of albumin-binding modification — and it is what turns a short-acting GHRH peptide into a long-acting one.
How PK drives dosing frequency
The cleanest way to see PK in action is to compare a short-acting and a long-acting peptide that target the same pathway (the growth hormone axis):
- Short-acting GHRPs / GHRH peptides — e.g. Ipamorelin, Hexarelin, Sermorelin, and CJC-1295 without DAC. These have half-lives on the order of minutes and produce a brief pulse of growth hormone. Because the effect is over quickly, protocols often dose multiple times per day, frequently timed around sleep or away from meals to mimic the body’s natural pulses.
- Long-acting, albumin-bound — CJC-1295 with DAC. The albumin-binding modification stretches the half-life to roughly a week, so a single dose keeps growth-hormone-stimulating activity elevated for days. That allows once- or twice-weekly dosing, but it raises baseline GH/IGF-1 in a sustained way rather than in discrete pulses.
The trade-off is a recurring theme: short half-life means more frequent dosing but more “physiological,” pulsatile signaling; long half-life means convenience and steadier levels but a more constant, less natural signal. The same logic explains why approved metabolic peptides like Semaglutide (long half-life, weekly) are dosed so differently from minute-to-minute-acting research peptides.
PK is not a dosing recommendation
Half-life explains the shape of a protocol — it does not tell you a safe or effective dose, and the numbers in the Half-Life Reference Chart are approximate and vary by individual, formulation, and route. This page is educational only and not medical advice; see the Disclaimer.
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