Half-life (t½) is the time it takes for the amount of a peptide in the body to fall by half. It explains why some peptides are given once a week and others several times a day, how long effects last, and how levels build up with repeated doses. This guide covers the key terms, walks through a worked tirzepatide example, and ends with a reference table of reported half-lives.
Key takeaways
- Short half-life means more frequent dosing; long half-life means less frequent dosing.
- With repeated doses, levels build up and level off at steady state after about 4–5 half-lives.
- Splitting the same weekly total into smaller, more frequent doses does not change average exposure. It narrows the gap between peaks and troughs.
- Half-life is always an approximation. A range is more accurate than a single number.
- This is pharmacokinetic math, not dosing guidance.
Key terms
- Half-life: how fast a peptide is cleared from circulation.
- Steady state: the point where the amount cleared between doses equals the amount added by each dose, so levels stop climbing and repeat the same pattern each cycle.
- Time to steady state: how long it takes to reach that stable pattern after a regimen starts. It is about 4–5 half-lives for any compound, but it only becomes noticeable with long-acting peptides, where it can take weeks.
- Onset of action: how quickly an effect appears.
- Duration of action: how long the effect lasts. This can outlast the half-life. Sermorelin, for example, has a half-life of roughly 10–12 minutes, but the growth hormone release it triggers, and the IGF-1 that follows, last for hours. Duration data is not always published, but when it is, it is very useful context.
Why half-life matters
Dosing schedule
- Short half-life = more frequent dosing.
- Long half-life = less frequent dosing.
Type of effect
- Short-acting peptides tend to mimic the body’s natural pulses.
- Long-acting peptides keep receptors engaged more steadily.
Steady state and safety
- With repeated dosing, levels build up and plateau at steady state, usually after 4–5 half-lives. Changes made before that point are judged on levels that are still rising.
Worked example: tirzepatide once weekly
Tirzepatide has a half-life of about 5 days after subcutaneous injection, and its FDA label starts at 2.5 mg once weekly. After 7 days, about 38% of a dose is still present (0.57/5 ≈ 0.38). Each new injection adds to what is left, so levels climb until the amount cleared each week equals the 2.5 mg added. With a 5-day half-life, that takes about 20–25 days.
| Week | Left from earlier doses | After injecting 2.5 mg |
|---|---|---|
| 1 | 0 mg | 2.50 mg |
| 2 | ~0.95 mg | ~3.45 mg |
| 3 | ~1.31 mg | ~3.81 mg |
| 4 | ~1.44 mg | ~3.94 mg |
| Steady state | ~1.52 mg | ~4.02 mg |
Same weekly total, split twice weekly
As a math illustration only (the label schedule is once weekly), here is the same 2.5 mg per week given as 1.25 mg every 3.5 days. After 3.5 days, about 62% of a dose is still present (0.53.5/5 ≈ 0.62).
| Day | Left from earlier doses | After injecting 1.25 mg |
|---|---|---|
| 0 | 0 mg | 1.25 mg |
| 3.5 | ~0.77 mg | ~2.02 mg |
| 7 | ~1.24 mg | ~2.49 mg |
| 10.5 | ~1.53 mg | ~2.78 mg |
| 14 | ~1.71 mg | ~2.96 mg |
| 21 | ~1.89 mg | ~3.14 mg |
| 28 | ~1.96 mg | ~3.21 mg |
| Steady state | ~2.00 mg | ~3.25 mg |
Weekly vs split: what changes
| Schedule | Steady-state peak | Steady-state trough | Average amount |
|---|---|---|---|
| 2.5 mg once weekly | ~4.0 mg | ~1.5 mg | ~2.6 mg |
| 1.25 mg twice weekly | ~3.25 mg | ~2.0 mg | ~2.6 mg |
Same weekly total, same average exposure, different pattern. Weekly dosing swings between higher peaks and lower troughs. Split dosing gives a flatter, steadier curve. Both reach steady state in about the same time, because that depends on half-life, not on how the dose is divided.
These numbers use a simplified model that treats each dose as entering the body all at once. In reality, subcutaneous tirzepatide is absorbed over one to three days, which smooths the peaks somewhat. The pattern still holds.
Why half-life values can differ
You may see the same peptide listed as “~5 days” in one place and “~5–6 days” in another. That is normal. Half-life varies because of:
- Different people: age, weight, kidney function and metabolism all change clearance.
- Study design: some trials sample longer or use more sensitive assays.
- Math models: researchers fit the data in different ways.
- Route and formulation: injection site, stabilizers and excipients can change absorption.
The bottom line: half-life is always an approximation, and a range is more accurate than one fixed number.
Reference half-life guide
Values are for subcutaneous or intramuscular use unless noted. Several entries rest on small studies or animal data, so treat them as rough guides.
Long-acting (days)
| Compound | Reported half-life |
|---|---|
| CJC-1295 with DAC | ~5.8–8.1 days |
| Semaglutide | ~7 days |
| Tirzepatide | ~5 days |
| Retatrutide | ~6 days |
| Cagrilintide | ~6–8 days |
Intermediate (hours)
| Compound | Reported half-life |
|---|---|
| HCG | ~24–36 h (varies by product and route) |
| IGF-1 LR3 | ~20–30 h (mostly preclinical data) |
| MK-677 (oral; not a peptide) | ~4–6 h (GH/IGF-1 effect lasts ~24 h) |
| SS-31 (elamipretide) | ~3–4 h |
| PT-141 (bremelanotide) | ~2.7 h |
| GHRP-6 | ~2–3 h (limited data) |
| Ipamorelin | ~2 h |
| Thymosin Alpha-1 | ~2 h |
| Thymosin Beta-4 / TB-500 fragment | ~1–2 h (limited data) |
| Hexarelin | ~55–70 min |
Short-acting (minutes)
| Compound | Reported half-life |
|---|---|
| Tesamorelin | ~26–38 min |
| CJC-1295 without DAC (Mod GRF 1-29) | ~30 min |
| GHRP-2 | ~30 min |
| Gonadorelin | ~10–40 min |
| Epitalon (Epithalon) | ~20 min (reported) |
| Glutathione | ~10–15 min |
| Oxytocin | ~10–15 min |
| Sermorelin | ~6–12 min |
| DSIP | ~7–8 min |
| GLP-2 (native) | ~5–7 min |
| Kisspeptin-10 | ~4 min |
| VIP | ~1–2 min |
Not well characterized in humans
These have only animal, early research or no published human half-life data.
| Compound | What is known |
|---|---|
| 5-Amino-1MQ | No published human data |
| AOD-9604 | ~3 min IV in animals |
| ARA-290 | ~20 min SC (limited reports) |
| BPC-157 | Under 30 min in animals; human data unknown |
| Cerebrolysin | Mixture; no single half-life |
| Dihexa | Preclinical only |
| FOXO4-DRI | No published human data |
| GHK-Cu | ~30–60 min (reported) |
| KPV | No published human data |
| LL-37 | ~1 h (preclinical) |
| Melanotan I | Peptide itself is short-lived; implant releases over ~30 days |
| Melanotan II | ~2–4 h (not well defined in humans) |
| MGF | Very short; PEG-MGF extends it |
| MOTS-c | No published human data |
| NAD+ | Not a peptide; minutes to ~1 h depending on tissue |
| PE 22-28 | No published human data |
| Pinealon | No published human data |
| Selank | Intranasal kinetics not standardized |
| Thymalin | Mixture; no single half-life |
FAQ
Does a longer half-life mean a peptide is stronger?
No. Half-life describes how long a compound stays in circulation, not how potent it is.
Why does it take weeks to feel the full effect of a weekly GLP-1?
Levels keep climbing until steady state, which takes about 4–5 half-lives. For a 5–7 day half-life, that is roughly 3–5 weeks at each dose level.
Does splitting a weekly dose change how much is in the body?
Not on average. The same weekly total gives the same average exposure. Splitting lowers the peaks and raises the troughs.
Why do short half-life peptides sometimes work for hours?
Some trigger downstream effects, such as hormone release, that last longer than the peptide itself. That is duration of action, which is separate from half-life.
For reconstitution and syringe math, see Peptide Math. For how doses are actually set, see where dosing numbers come from.
Medical disclaimer: Pep Guide is for educational purposes only. It is not medical advice, diagnosis or treatment. Research changes over time. Talk to a licensed healthcare provider before starting, stopping or changing any medication or compound.