UNDERSTANDING HALF-LIVES AND DOSING WINDOWS
Half-life is the most important pharmacokinetic concept for peptide users. It determines dosing frequency, injection timing logic, how to read blood test results, and how long after stopping a compound its effects persist.
1.Half-life fundamentals
A compound's half-life is the time required for its plasma concentration to fall by 50%. After one half-life, 50% of the original concentration remains. After two half-lives, 25%. After three, 12.5%. After five half-lives, approximately 3% remains — which is considered pharmacologically negligible (effectively cleared). This exponential decay applies to every compound and can be used to predict concentration at any time after a dose.
For peptides, which are generally cleared more rapidly than small-molecule drugs (due to proteolytic degradation in plasma and tissues), half-lives range from minutes (some GHRPs) to days (semaglutide). The half-life determines everything about dosing frequency: a compound with a 30-minute half-life must be dosed multiple times daily to maintain effect; one with a 7-day half-life achieves steady state with once-weekly dosing.
Steady state: when you dose a compound repeatedly, plasma concentration rises with each dose until the rate of clearance equals the rate of input. This equilibrium — called steady state — is reached after approximately 4-5 half-lives of consecutive dosing. For semaglutide (7-day half-life), steady state is achieved after 4-5 weeks of weekly dosing. For CJC-1295 no DAC (30-min half-life), steady state within a single injection-to-clearance cycle.
Biological half-life vs. plasma half-life: the time a compound remains in plasma is the plasma half-life. The biological half-life is the duration of pharmacological effect, which is often longer. BPC-157 has a plasma half-life of approximately 60-90 minutes — it clears from blood quickly. Yet its healing effects persist for 24+ hours per dose because the downstream changes it initiates (gene expression changes, growth factor upregulation, VEGF activation) persist far beyond the peptide itself.
Accumulation with repeated dosing: compounds with longer half-lives accumulate with each dose until steady state. Semaglutide's effective concentration at week 5 of weekly dosing is approximately 2x the concentration after the first dose. This accumulation is why GLP-1 side effects often worsen slightly in weeks 3-5 before the body adapts — peak concentration is still rising through that period.
2.Short half-life peptides: the pulsatile class
CJC-1295 no DAC (Modified GRF 1-29) has a plasma half-life of approximately 30 minutes. This is actually the pharmacokinetically ideal property for a GHRH analog: it produces a brief, intense GHRH signal that mimics the natural hypothalamic pulse rather than a sustained elevation that would suppress pituitary sensitivity. Its short half-life is a feature, not a limitation.
Ipamorelin has a plasma half-life of approximately 2 hours. It needs to be dosed within the injection window before sleep or morning fasted state to effectively capture the target GH pulse. The 2-hour half-life means it is still partially active 4-6 hours after injection — long enough to extend its effect into the deep sleep GH release phase.
GHRP-2 and GHRP-6 have half-lives in the 15-30 minute range for the initial rapid clearance phase, with a longer tail. Both are typically dosed immediately before the target injection window (pre-sleep, pre-workout) with the understanding that the primary GH pulse effect occurs within the first 60 minutes.
The short half-life compounds collectively: each injection is essentially an isolated pharmacological event. Missing a single injection does not produce a multi-day gap in the protocol because there is no meaningful accumulation from previous doses to buffer the missed dose. Consistency matters more for short-half-life compounds than for long-half-life ones.
BPC-157 has a plasma half-life of approximately 60-90 minutes in rodent models (human data is limited). Once-daily injection is effective because the biological effects it initiates — gene expression changes, growth factor activation, angiogenic signaling — persist for 18-24 hours after the peptide itself has cleared. The molecular cascade is the drug, not the peptide molecules themselves at steady state.
3.Long half-life peptides: sustained concentration
CJC-1295 with DAC (drug affinity complex) incorporates a molecular modification that enables the peptide to bind to serum albumin, dramatically extending its plasma half-life to approximately 8 days. A single injection maintains meaningful GHRH receptor stimulation for over a week. This fundamentally changes the pharmacological pattern: instead of pulsatile GHRH stimulation, you get sustained GHRH elevation.
The DAC vs no-DAC debate: sustained GHRH elevation (from CJC with DAC) blunts natural GH pulsatility by providing continuous receptor stimulation that prevents the post-pulse receptor resensitization window. This may contribute to faster receptor desensitization over a cycle compared to pulsatile stimulation. Most protocols favor CJC no DAC for this reason, trading the convenience of once-weekly injections for more physiologically appropriate pulsatile signaling.
Semaglutide's 7-day half-life is the basis for weekly dosing. At steady state (approximately 4-5 weeks of weekly dosing), trough concentrations (just before the next weekly injection) are approximately 60-70% of peak concentrations. This creates fairly stable plasma levels — very different from the discrete injection pulses of GHRPs. The steady concentration is part of what makes GLP-1 agonists effective at sustained appetite regulation.
Tirzepatide's half-life of approximately 5 days allows once-weekly dosing with slightly more variation in peak-to-trough ratio than semaglutide. By week 5-6 of weekly dosing, tirzepatide reaches steady-state concentrations that are approximately 2.5x the single-dose concentration. This accumulation is why early dose escalation often produces more side effects than the same dose at steady state weeks later.
Practical implication of long half-lives: missing a dose has less immediate impact because previous doses contribute significant residual concentration. If you miss a weekly semaglutide injection, your effective concentration on day 8 is roughly 50% of usual trough — meaningfully present, not absent. Skipping a week entirely on a GH peptide with a 30-minute half-life produces essentially zero effect by the morning after the missed injection.
4.Half-life and blood testing timing
Testing IGF-1 to assess GH peptide protocol efficacy: IGF-1 reflects cumulative GH activity over several days to a week, not a single injection event. The timing of your last injection relative to the blood draw is not critical for IGF-1 — you do not need to inject the morning of the test or avoid injecting for 24 hours before. Draw at your normal waking time whenever is convenient for the lab appointment.
Testing fasting glucose mid-protocol: should be done in the morning after an 8+ hour fast, regardless of when you last injected your GH peptide. Fasting glucose measures metabolic status, not the acute pharmacological effect of the last dose. It is not acutely changed by a single injection the night before.
GLP-1 trough testing: if you want to measure the minimum (trough) plasma concentration of semaglutide or tirzepatide to confirm adequate dosing, draw blood on day 6-7 after your last injection — the day just before your next scheduled weekly dose. This trough value is the most clinically relevant reference point for assessing consistent dosing.
The timing trap to avoid: drawing IGF-1 at peak (within 2-3 hours of an injection) gives you an artificially high result from acute GH secretion rather than the sustained IGF-1 production you are trying to measure. Draw IGF-1 at trough (24+ hours after your last injection) for the most meaningful assessment of baseline GH axis activity during your protocol.
Cortisol testing for GHRP-2 users: GHRP-2 elevates cortisol significantly. If you are using GHRP-2 and want to assess cortisol elevation, draw in the morning approximately 30-60 minutes after your morning injection to capture the post-injection peak. Compare to a pre-protocol fasting morning cortisol (the standard reference) to quantify the elevation.
Sources & Studies
Raun K. et al., Eur J Endocrinol, 1998
Dychter SS. et al., J Infus Nurs, 2012