Peptide half-lives explained
Half-life is the single most important pharmacokinetic number on a peptide data sheet. It determines how often to dose, when effects stabilize, and how much accumulation you should expect. This guide explains the math, the intuition, and the practical dosing rules in plain language — no textbook required.
In this guide
What half-life actually means
Half-life (abbreviated t½) is the time it takes for the plasma concentration of a substance to fall by 50% after it has been absorbed. It is an elimination metric — how quickly your body clears the compound — and it is remarkably consistent across a dose range (which is why it is useful as a single number).
Concrete example. A peptide with a 10-hour half-life, injected at a dose that produces a 100 ng/mL peak plasma concentration, will roughly follow this curve:
| Time after dose | Plasma level | % of peak |
|---|---|---|
| 0 hours | 100 ng/mL | 100% |
| 10 hours (1 half-life) | 50 ng/mL | 50% |
| 20 hours (2 half-lives) | 25 ng/mL | 25% |
| 30 hours (3 half-lives) | 12.5 ng/mL | 12.5% |
| 40 hours (4 half-lives) | 6.25 ng/mL | ~6% |
| 50 hours (5 half-lives) | 3.1 ng/mL | ~3% |
After 5 half-lives, typically less than 5% of the dose remains — functionally "gone" for most practical purposes. This is the source of the "5 half-lives" rule you see repeated everywhere: it is the time to near-complete washout after stopping, and it is also how long steady-state takes to build up (see below).
You can play with this visually in the half-life visualizer — set the half-life, interval, and dose, and watch the plasma curve in real time.
Choosing a dosing cadence
The practical question half-life answers: how often should I inject?
There is not one right answer, but there are three dominant strategies, each suited to a different peptide class:
1. Dose frequency ≈ half-life (stable plasma)
Dosing at roughly the half-life interval produces a stable plasma level that oscillates between peak and ~50% trough. This is the default for peptides where continuous receptor engagement is desirable — most GLP-1s, growth hormone, many therapeutic compounds.
2. Dose frequency < half-life (accumulation / smoothing)
Dosing more frequently than the half-life produces a smoother curve with less peak-to-trough variation, and higher average plasma level via accumulation. Common with fast-onset peptides that benefit from constant presence.
3. Dose frequency > half-life (pulsatile)
Dosing less often than the half-life allows plasma to fall to near-zero between doses. This is desirable for GHRH and GHRP compounds, where the pulse pattern mimics endogenous growth hormone secretion — continuous exposure would actually desensitize the receptor and reduce efficacy. Sermorelin, Ipamorelin, and short-acting CJC-1295 are dosed this way intentionally.
Steady-state: when the protocol stabilizes
On repeated dosing, plasma levels do not stay flat — they rise over several doses until the amount cleared per interval equals the amount dosed. That plateau is steady-state.
This holds regardless of dose, regardless of interval, regardless of route of administration. It is a property of the elimination kinetics.
| Peptide | Half-life | Time to steady-state |
|---|---|---|
| Sermorelin | ~10 min | ~50 min (irrelevant — used pulsatile) |
| BPC-157 | ~4 h | ~20 h |
| Ipamorelin | ~2 h | ~10 h |
| HGH | ~4 h (subq) | ~20 h |
| Tesamorelin | ~25 min | Not meaningful — used once daily with washout |
| CJC-1295 with DAC | ~200 h | ~6 weeks |
| Tirzepatide | ~120 h | ~4 weeks |
| Semaglutide | ~165 h | ~5 weeks |
| Retatrutide | ~165 h (reported) | ~5 weeks |
The practical implication is substantial: effects you attribute to the peptide are often not fully expressed until steady-state. If you are evaluating a semaglutide dose after two weeks, you are only halfway to a stable level. Wait at least 4–6 weeks before declaring a dose insufficient.
Accumulation factor
Accumulation factor is the ratio of steady-state peak to single-dose peak. It quantifies how much higher your plasma runs after repeated dosing compared to a single injection.
Reading this without getting lost in the math:
- If interval = half-life: accumulation factor = 2× (steady-state peak is twice the single-dose peak).
- If interval = ½ half-life (dosing twice as often): accumulation ≈ 3.4×.
- If interval = 2 × half-life (dosing half as often): accumulation ≈ 1.3×.
- If interval >> half-life (pulsatile): accumulation ≈ 1× (no meaningful accumulation).
Practical examples:
- Semaglutide (t½ ~165 h, dosed weekly/168 h): accumulation ≈ 2×.
- CJC-1295 with DAC (t½ ~200 h, dosed weekly): accumulation ≈ ~1.7×.
- Ipamorelin (t½ ~2 h, dosed twice daily at 12 h): accumulation ≈ 1.02× — essentially none.
The half-life visualizer computes accumulation factor automatically for any peptide × interval combination.
Half-life table for common peptides
| Peptide | Half-life | Typical cadence |
|---|---|---|
| Sermorelin | ~10 min | Once nightly — pulsatile |
| Ipamorelin | ~2 h | 1–3× daily, pulsatile |
| GHRP-2 / GHRP-6 | ~20–30 min | 1–3× daily, pulsatile |
| Hexarelin | ~70 min | 1–2× daily |
| CJC-1295 (no DAC) | ~30 min | 1–3× daily, pulsatile |
| CJC-1295 with DAC | ~200 h (~8 d) | Once weekly |
| Tesamorelin | ~25 min | Once daily (nightly) |
| BPC-157 | ~4 h | 1–2× daily |
| TB-500 | ~2–3 h (loading); weeks (tissue) | Twice weekly |
| GHK-Cu | ~30 min (plasma) | Daily topical or subq |
| PT-141 | ~2 h | On-demand, not scheduled |
| MT-II | ~1 h | Daily during loading, then weekly maintenance |
| HGH | ~4 h (subq) | Once daily (AM or PM) |
| IGF-1 LR3 | ~20–30 h | Once daily |
| HCG | ~36 h | 2–3× per week |
| Semaglutide | ~165 h (7 d) | Once weekly |
| Tirzepatide | ~120 h (5 d) | Once weekly |
| Retatrutide | ~165 h (reported) | Once weekly |
| Thymosin Alpha-1 | ~2 h | Twice weekly |
| Epithalon | ~20 min | Short daily course then washout |
For per-peptide half-life visualizations and PK stats, see the half-life visualizer hub.
Short vs long: the trade-off
Longer half-life peptides are not universally "better" — they trade one set of properties for another.
| Short half-life (< 6 h) | Long half-life (> 24 h) | |
|---|---|---|
| Convenience | Multiple daily doses | Weekly or less |
| Plasma curve | Peak-trough pattern; pulsatile possible | Smooth, flat |
| Side-effect control | Problems resolve quickly on dose reduction | Problems persist for days to weeks after stopping |
| Receptor dynamics | Allows receptor recycling between doses | Continuous occupancy — can cause desensitization |
| Accumulation risk | Minimal | Significant if dose interval is short |
| Missed dose impact | Immediate drop in plasma | Barely noticeable |
For GHRH/GHRP peptides specifically, the pulsatile pattern of short-half-life compounds is a feature, not a bug — it more closely mimics endogenous growth hormone secretion. This is why Sermorelin (t½ ~10 min) and Ipamorelin (t½ ~2 h) remain relevant despite the existence of longer-lived alternatives.
Practical dosing rules of thumb
- For stable-plasma peptides (GLP-1s, HGH, most therapeutics): dose interval at or slightly below half-life.
- For pulsatile peptides (GHRH/GHRP): dose interval well above half-life, aligned to desired receptor rhythm.
- Before evaluating a dose: wait at least 5 half-lives for steady-state. This means ~5 weeks for weekly GLP-1s, ~2 weeks for daily IGF-1 LR3, a single day for twice-daily BPC-157.
- When changing doses: expect the new level to stabilize over 5 half-lives of the new regimen, not immediately.
- When stopping: effects fade over 5 half-lives. For long-half-life compounds, this can be weeks — plan accordingly if pregnancy, surgery, or labs are upcoming.
Frequently asked questions
What does a peptide half-life mean?
The time it takes for plasma concentration to fall by 50% after absorption. A 10-hour half-life means plasma drops to 50% in 10 hours, 25% in 20 hours, and so on.
How does half-life determine dosing frequency?
As a rough rule, dose at least as often as the half-life for stable plasma, or much less often for pulsatile strategies. Very long half-lives (semaglutide ~165 h) permit weekly cadences.
What is steady-state and how long until I reach it?
The point at which the amount of peptide entering the body per interval equals the amount cleared. Reached after ~5 half-lives of consistent dosing — from hours (short peptides) to weeks (semaglutide).
What is accumulation factor?
The ratio of steady-state peak to single-dose peak. When interval = half-life, it is 2×. When interval is much longer than half-life, it is ~1× (no accumulation).
Do I dose more often for shorter half-lives?
Generally yes, if a stable plasma level is the goal. Some short-half-life peptides are intentionally dosed pulsatile to match endogenous rhythms.
Is longer half-life always better?
No. Longer means convenience but slower side-effect resolution and continuous receptor occupancy — which can cause desensitization with some mechanisms.
Visualize your protocol's plasma curve
Use the interactive half-life visualizer to chart steady-state, accumulation, and peak-to-trough ratios for any peptide × cadence combination.
Open half-life visualizer →Related guides
Educational use only. Pharmacokinetic values cited are approximations from published literature and vary by route of administration, formulation, and individual. This guide is not medical advice.