Half-life numbers look intimidating on a compound page, but the whole concept comes down to one practical question: how often should I dose this? Here’s how to read a half-life and translate it into a frequency that makes sense.
What “half-life” actually means
A compound’s half-life is the time it takes for the body to eliminate half of what’s currently circulating. After one half-life, 50% of the dose is gone. After two half-lives, 75%. After three, 87.5%. After four, about 94%.
It’s not a clock that resets — it’s a percentage rule. So a compound with a 4-hour half-life doesn’t disappear in 4 hours; it just halves every 4 hours, asymptotically approaching zero.
The four-half-lives rule
Most dosing protocols are designed around two competing goals:
- Maintain a useful plasma concentration — you need enough of the compound in circulation to engage its target.
- Avoid runaway accumulation — dosing faster than the body clears means levels stack up across days and eventually overshoot.
Half-life sets the balance point. If you re-dose around one half-life later, you maintain a roughly steady plasma level. If you wait four or five half-lives between doses, you’re letting the compound clear almost completely between hits — which suits short-acting compounds where you actually want a peak-and-trough pattern.
A short half-life vs a long one — visualised
Translating half-life into a dosing schedule
Re-dose interval ≈ half-life → steady plasma concentration
Re-dose interval ≈ 4 × half-life → peak-and-trough pattern, near-complete clearance
A few worked examples:
| Compound | Half-life | Common dosing in research | Why |
|---|---|---|---|
| BPC-157 | ~4 hours | Once or twice daily | Clears almost fully overnight — daily dosing produces a fresh peak each day rather than accumulating |
| TB-500 | ~48 hours | Twice weekly | Daily dosing accumulates faster than receptor signalling can use; 2×/week maintains steady levels |
| Semaglutide | ~7 days | Once weekly | Half-life slightly longer than the dosing interval, so plasma sits at near-steady state |
| Retatrutide | ~6 days | Once weekly | Same logic as Semaglutide — weekly dosing matches the elimination window |
| IGF-1 LR3 | ~20–30 hours | Once daily | Long enough to bridge 24 hours, short enough that daily dosing doesn’t pile up |
| Ipamorelin | ~2 hours | 2–3 times daily | Short pulse with full clearance — needed to trigger natural GH-pulse pattern |
Why some compounds break the rule
The half-life → frequency mapping works for most peptides, but a few categories deliberately break it:
Pulsatile-signalling compounds
GHRPs (Ipamorelin, GHRP-2, GHRP-6) work by triggering a pituitary pulse, not by sitting in plasma. They’re dosed several times a day even though their half-lives could support longer intervals — because the goal is repeated signal pulses, not maintained concentration.
Compounds with long downstream effects
Tesamorelin has a plasma half-life of ~25 minutes but is dosed once daily. The GH pulse it triggers and the IGF-1 elevation that follows last hours, so the practical effect outlives the parent compound by a wide margin.
Receptor-saturation compounds
Some compounds saturate their receptors at far lower concentrations than peak plasma levels. Once the receptors are saturated, additional plasma concentration doesn’t add effect. For these, dosing schedule is set by receptor recycling time, not plasma clearance.