Peptide half-life calculator
Calculate remaining active peptide and optimal redose interval using published pharmacokinetic half-life data. Covers BPC-157, tirzepatide, retatrutide, TB-500, CJC-1295, ipamorelin, and more.
Half-life is the time required for the concentration of a compound in the body to decrease by 50%. It is the fundamental pharmacokinetic parameter that determines how frequently a peptide must be administered to maintain therapeutic levels and avoid accumulation.
Peptide half-lives vary enormously by molecular structure and administration route. Sermorelin has a plasma half-life of approximately 10 minutes — it is cleared so rapidly that its effects are primarily through the initial GH pulse it triggers. CJC-1295 without the Drug Affinity Complex (DAC) has a half-life of 30 minutes. With DAC attached, the same molecule has a half-life of approximately 8 days because the DAC modification causes it to bind albumin in the bloodstream, dramatically slowing clearance.
Tirzepatide and retatrutide have half-lives of approximately 5 and 6 days respectively — which is precisely why they are administered once weekly. A once-weekly schedule maintains steady-state plasma levels with minimal peak-to-trough variation. TB-500 has a 5-day half-life and is typically dosed twice weekly during an acute healing phase, then once weekly for maintenance.
The 2× half-life rule for redosing is a standard pharmacokinetic guideline: the redose interval should be approximately 2× the half-life to maintain therapeutic levels. This results in dosing when approximately 25% of the previous dose remains active — preventing both accumulation and significant gaps in coverage.
This calculator uses exponential decay mathematics with published half-life values for 10 common research peptides.
Half-life: 4 hours
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50%
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125 mcg
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8 hours
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Frequently asked questions
What is the half-life of tirzepatide and why is it dosed once weekly?+
Tirzepatide has a plasma half-life of approximately 5 days (120 hours) via subcutaneous injection. This extended half-life — achieved through structural modifications including a C20 fatty acid chain for albumin binding — allows once-weekly dosing while maintaining stable plasma concentrations. At weekly dosing, plasma levels reach steady state after approximately 4 weeks (approximately 5 half-lives).
How often should I inject BPC-157 based on its half-life?+
BPC-157 has a half-life of approximately 4 hours via subcutaneous injection. Using the 2× half-life rule, the redose interval is approximately 8 hours — meaning twice-daily dosing (every 8-12 hours) is the standard. Morning and pre-sleep injections are the most common timing for twice-daily BPC-157 protocols.
What is the half-life difference between CJC-1295 with and without DAC?+
CJC-1295 without DAC (Drug Affinity Complex) has a half-life of approximately 30 minutes. CJC-1295 with DAC has a half-life of approximately 8 days (192 hours). The DAC modification causes the molecule to bind to albumin in the bloodstream, dramatically slowing renal clearance. Without DAC produces a pulsatile GH release pattern; with DAC produces sustained elevated GH levels. These are fundamentally different pharmacological profiles.
Why does half-life matter for peptide research dosing frequency?+
Half-life determines how much active compound remains at any given time after injection. Dosing too infrequently relative to half-life creates troughs where minimal active compound is present — reducing efficacy. Dosing too frequently relative to half-life causes accumulation, which can increase both desired effects and adverse effects. Correct dosing frequency matched to half-life maintains consistent therapeutic plasma levels.
What does the 2× half-life redose rule mean in practice?+
The 2× half-life rule states that to maintain therapeutic levels without significant accumulation, the redose interval should be approximately twice the half-life. At this interval, approximately 25% of the previous dose remains when the new dose is administered. Examples: sermorelin (10 min half-life) — redose every 20 minutes if continuous effect is desired; ipamorelin (2h half-life) — redose every 4 hours for continuous coverage; tirzepatide (120h half-life) — redose every 240 hours = once weekly.
Research background
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