Why Half-Life Matters for Study Design
Half-life (t½) is the time required for plasma concentration of a compound to fall to 50% of its initial value. In research contexts, half-life determines:- Dosing interval: How frequently compound must be administered to maintain target exposure windows in in vivo studies
- Washout period: How long before an experiment or measurement to allow compound to clear sufficiently for baseline or crossover designs
- Steady-state timing: How long repeat-dosing continues before plasma levels reach a stable plateau (approximately 4–5 half-lives)
- Pulsatile vs. sustained profiles: Short half-life compounds produce discrete spikes; long half-life compounds produce sustained plateau-like exposure
This reference summarizes published plasma half-life values for research peptides stocked by Phase 1 Peptides. Values represent published estimates from pharmacokinetic studies in rodent or human models and may vary by species, route, formulation, and individual model.
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GLP-1 / Metabolic Peptides
| Compound | Half-Life | Dosing Model in Research | Notes |
| Tirzepatide | ~5 days | Once weekly (in vivo) | GIP/GLP-1 dual agonist; C20 fatty diacid for albumin binding |
| Retatrutide | ~6–7 days | Once weekly | GLP-1/GIP/GCGR triple agonist; estimated from Phase 2 data |
| Semaglutide | ~7 days | Once weekly | Aib substitution + C18 fatty diacid; most published once-weekly GLP-1 data |
| Mazdutide | ~7 days | Once weekly | GLP-1/GCGR dual agonist; estimated from Phase 2/3 data |
| Cagrilintide | ~7 days | Once weekly | Amylin analog; C20 fatty acid for albumin binding |
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GHRH Analogs (Growth Hormone Releasing Hormone)
| Compound | Half-Life | Dosing Model in Research | Notes |
| Sermorelin | 10–20 minutes | Short-pulse, multi-dose per day | Native GHRH(1-29) sequence; rapid GHRH receptor agonism |
| CJC-1295 No DAC | ~30 minutes | 2–3× daily pulse | Stabilized GHRH(1-29); longer than sermorelin but still pulsatile |
| CJC-1295 with DAC | 6–8 days | Once or twice weekly | Drug Affinity Complex binds albumin; sustained GHRHR occupancy |
| Tesamorelin | ~35–40 minutes | Once daily | Full 44-aa GHRH sequence + trans-3-hexenoic acid N-terminal modification |
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GHRPs (Growth Hormone Releasing Peptides)
| Compound | Half-Life | Dosing Model in Research | Notes |
| Ipamorelin | ~2 hours | Pulse dosing (1–3× daily in animal models) | Selective GHS-R1a agonist; minimal cortisol/prolactin |
| GHRP-6 | 15–60 minutes | Frequent pulse | Significant cortisol and prolactin effects |
| GHRP-2 | ~30–60 minutes | Frequent pulse | Stronger GH release than GHRP-6; cortisol/prolactin effects present |
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Neuropeptides
| Compound | Half-Life | Dosing Model in Research | Notes |
| Semax | ~15–30 minutes | Multi-dose per day; intranasal in published studies | ACTH(4-10) analog; rapid CNS access reported |
| Selank | ~2 minutes (plasma); CNS effects longer | Multi-dose per day; intranasal | Rapid proteolytic clearance; CNS effect duration not strictly coupled to plasma t½ |
| DSIP | ~30–60 minutes | Multi-dose; variable by model | Apparent BBB penetration intact; CNS distribution complicates PK interpretation |
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Tissue Repair and Structural Peptides
| Compound | Half-Life | Dosing Model in Research | Notes |
| BPC-157 | ~30–60 minutes (animal models) | Typically once or twice daily in rodent models | Stable in gastric acid; both IV and oral administration in published studies |
| TB-500 | Not precisely characterized | Typically once or twice weekly in longer-course rodent studies | Slow tissue distribution may extend effective half-life beyond plasma clearance |
| GHK-Cu | Not precisely characterized | Variable; in vitro typically media concentration | Tripeptide; rapid plasma clearance expected but tissue retention under study |
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Mitochondrial / Longevity Compounds
| Compound | Half-Life | Dosing Model in Research | Notes |
| MOTS-c | Not fully characterized; minutes to hours | Typically 3–5× weekly in rodent longevity studies | Mitochondrial-derived peptide; tissue distribution kinetics under investigation |
| SS-31 (Elamipretide) | ~2 hours (human clinical data available) | Once daily in cardiac/renal animal models | Direct cardiolipin-binding mechanism; clinical PK data from Phase 2/3 trials |
| NAD+ | Minutes (exogenous IV); intracellular turnover hours | Variable by administration route and study design | Exogenous NAD+ rapidly converted; intracellular pool kinetics the relevant measure |
| Glutathione (GSH) | Minutes (plasma); intracellular GSH pools hours | In vitro: media concentration; in vivo: usually local | Tripeptide; plasma clearance rapid but intracellular synthesis/recycling ongoing |
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Small Molecules (Non-Peptide, Research Context)
| Compound | Half-Life | Dosing Model in Research | Notes |
| 5-Amino-1MQ | Not fully published; estimated hours | Once or twice daily in rodent adipose studies | Small-molecule NNMT inhibitor; cell-permeable; distributes to adipose/liver |
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Key Concepts for Half-Life Interpretation
Effective Half-Life vs. Plasma Half-Life
Plasma half-life measures compound disappearance from blood. Effective half-life (the duration of biological effect) may differ substantially — sometimes longer, sometimes shorter — depending on:
- Receptor binding kinetics (on-rate/off-rate)
- Internalization and recycling of receptor-ligand complexes
- Active metabolite formation
- Tissue accumulation and slow release
For neuropeptides like Selank, effective duration substantially exceeds plasma t½. For compounds with active metabolites, the opposite can apply.
Steady-State Accumulation
For any compound dosed more frequently than 5× its half-life allows, plasma levels accumulate. At steady state (reached after approximately 4–5 half-lives of repeat dosing), trough concentrations stabilize. This is relevant for long-half-life GLP-1 analogs: week 1 exposure is lower than week 5 exposure in a continuous dosing protocol.
Washout
For crossover designs, a compound's washout period is typically defined as ≥5 half-lives. For tirzepatide (t½ ~5 days), complete washout requires approximately 25 days. For a short-acting compound like GHRP-6 (t½ ~30 min), washout is complete in a few hours.
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Related Protocols
- How to reconstitute lyophilized peptides
- Peptide storage and stability
- Understanding your Certificate of Analysis
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Summary
Half-life is a core pharmacokinetic parameter governing study design: dosing intervals, washout periods, steady-state timing, and the distinction between pulsatile and sustained exposure profiles. GLP-1 analogs in this catalog operate on weekly timescales. GHRH analogs span minutes (sermorelin) to days (CJC-1295 DAC). GHRPs and neuropeptides operate on short pulse windows. Tissue repair and mitochondrial compounds are less precisely characterized but generally shorter-acting than the long-half-life metabolic peptides. Researchers designing multi-compound or crossover studies should account for the specific t½ of each agent to avoid confounding exposure overlap between treatment arms.
Frequently Asked Questions
What is peptide half-life, and why does it matter for research design?Half-life (t½) is the time required for the plasma concentration of a compound to decrease by 50%. In research, half-life governs dosing interval selection, washout period calculation, steady-state timing, and the distinction between pulsatile and tonic exposure profiles — all of which affect reproducibility across experimental arms.
Why do some peptides have very different biological half-lives versus plasma half-lives?Plasma t½ measures compound disappearance from blood. Biological half-life reflects the duration of receptor-mediated effect, which can differ substantially due to receptor binding kinetics, ligand internalization and recycling, active metabolite formation, or tissue accumulation. For certain neuropeptides, the biological effect outlasts the plasma measurement by several hours.
How many half-lives are needed for complete washout in a crossover study?Standard pharmacokinetic practice defines washout as ≥5 half-lives, at which point approximately 97% of the compound has been eliminated. For long-acting GLP-1 analogs (t½ 5–7 days), this requires 25–35 days. For short-acting GHRPs (t½ 20–30 min), washout is complete within a few hours.
What is steady-state concentration and when does it become relevant?When a compound is dosed repeatedly before complete elimination, plasma levels accumulate until a steady state is reached at approximately 4–5 half-lives of continuous dosing. For long-half-life compounds, early time points in multi-week protocols reflect lower exposure than later time points — a confound researchers should account for when interpreting longitudinal data.
See Also
- Lab Testing & Verified Purity
- GLP-1 Receptor Agonist Research Landscape
- GHRH Analog Comparison
- GHRP Comparison Overview
- Neuropeptide Research Overview
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