Half-life — what it means and why dosing depends on it

Half-life is the time it takes for half of a dose to be cleared from your body. It's the most important number on a peptide page after dose, because it dictates how often you inject and how soon you reach steady state.

The basic idea

Inject 1mg of a peptide with a 6-day half-life: - Day 0: 1mg in your system - Day 6: 0.5mg - Day 12: 0.25mg - Day 18: 0.125mg

After about 4-5 half-lives, the dose is effectively cleared (97%+ gone). Until you take another dose. Once you dose regularly enough that each new dose lands before the last one is fully cleared, you reach steady state — your body holds a roughly constant concentration of the peptide. Steady state is what produces the consistent effect; single doses don't.

Why half-life dictates dosing frequency

The dosing schedule on every peptide is reverse-engineered from its half-life. Two examples:

Ipamorelin half-life: ~2 hours. A single dose is mostly gone in 8-10 hours. To maintain effect, you have to dose 1-3 times daily. The standard pattern: morning, post-workout, pre-bed.

Retatrutide half-life: ~6 days. A single weekly dose covers the whole week with substantial overlap. By week 5-6 you're at steady state; the level you feel that week is what you'll feel every week from then on at that dose.

Daily injection of Ipa makes sense because of its short half-life. Daily injection of Reta would be insane — you'd build up massive accumulation and the side effects would be brutal.

What "steady state" actually feels like

For a long-half-life peptide (Reta, Tirz, Sema, Cagrilintide), the first 4-6 weeks are accumulation. The effects ramp during that window. People who quit at week 3 because "it's not doing anything" are quitting before steady state.

For a short-half-life peptide (Ipamorelin, GHRP-2, MGF), there's no real accumulation between doses. You either feel the dose or you don't. Steady state in the sense of accumulation doesn't apply; consistency comes from regular dosing.

How to read half-life on a peptide page

Pepdex shows half-life in the quick-facts strip. Some examples from across the catalog:

• BPC-157: ~4 hours sub-q (daily dosing, no accumulation)
• TB-500: ~2-3 days (twice weekly loading, weekly maintenance)
• Tirzepatide: ~5 days (once weekly, builds steady state)
• Retatrutide: ~6 days (once weekly, longer titration windows)
• Semaglutide: ~7 days (once weekly)
• HGH: ~3-5 hours injected (daily, sometimes twice daily)
• Ipamorelin: ~2 hours (1-3x daily)
• CJC-1295 no-DAC: ~30 min (1-3x daily, paired with Ipa)
• CJC-1295 with-DAC: ~8 days (different drug class — smooths the GH pulse rather than amplifying it)
• MT-1: ~1 hour (daily loading then 1-2x weekly maintenance)
• Octreotide LAR: 30 days (the slow-release IM depot — once monthly)

When half-life matters most for you

1. Picking dose frequency. If a protocol says "twice weekly" but the half-life is 6 hours, something is wrong with the protocol. Cross-check. 2. Understanding why titration takes weeks. Long-half-life drugs take 4-5 half-lives to reach steady state. That's why the GLP/GIP class titrates over 4-week steps. 3. Predicting when a cycle break clears the system. Stopping a peptide with a 6-day half-life means it's effectively cleared in ~30 days. A peptide with a 30-min half-life is cleared in hours. 4. Avoiding accumulation surprise. Long half-life = more stacking risk. If you switch from Ipamorelin to CJC-1295 with-DAC mid-cycle, the long DAC half-life means you have weeks of overlap to think about.

That's the whole concept. Half-life isn't an academic statistic; it's the math behind your protocol.