Understanding Peptide Half-Lives: Why Timing Matters

A half-life is the time it takes for the concentration of a compound in the body to decrease by 50%. After one half-life, 50% of the original amount remains active. After two half-lives, 25% remains. After three half-lives, 12.5%. After five half-lives, less than 3.2% of the original dose remains, which is generally considered the point at which the compound is effectively cleared. Half-life is one of the most important pharmacokinetic parameters because it directly determines how often a compound needs to be administered to maintain effective levels. A peptide with a 30-minute half-life requires multiple daily doses, while one with a 7-day half-life needs only weekly administration.

Many research peptides have remarkably short half-lives measured in minutes. BPC-157 has an estimated half-life of approximately 4 hours based on available preclinical data (PMID: 30915550). Modified GRF 1-29 (CJC-1295 without DAC) has a half-life of approximately 30 minutes (PMID: 17018654). Ipamorelin has a half-life of roughly 2 hours (PMID: 9849822). DSIP (Delta Sleep-Inducing Peptide) has a half-life of approximately 15-25 minutes. These short half-lives explain why protocols for these compounds typically involve multiple daily administrations — the compound is cleared quickly and levels drop below effective thresholds between doses.

Some compounds achieve extended half-lives through structural modifications or formulation technology. Semaglutide (Ozempic, Wegovy) has a half-life of approximately 7 days, achieved through albumin binding and DPP-4 resistance engineered into its molecular structure (PMID: 28930490). Tirzepatide (Mounjaro, Zepbound) has a similar half-life of approximately 5 days (PMID: 35658024). CJC-1295 with DAC has a half-life of 6-8 days due to the Drug Affinity Complex that enables albumin binding (PMID: 16352683). MK-677 (Ibutamoren), while technically not a peptide but a growth hormone secretagogue, has a half-life of approximately 6 hours, allowing once-daily oral dosing. These longer half-lives simplify protocol adherence since dosing is less frequent.

Understanding half-life is critical for protocol scheduling. For compounds with very short half-lives (under 1 hour), doses are typically timed 2-3 times daily to maintain more consistent levels. For moderate half-lives (2-6 hours), once or twice daily dosing is common. For long half-life compounds (days), weekly dosing is standard. The concept of 'steady state' is also important: when administering a compound at regular intervals, it takes approximately 4-5 half-lives to reach steady state, where the amount entering the system equals the amount being cleared. For semaglutide with a 7-day half-life, steady state is reached after approximately 4-5 weeks of weekly dosing.

Published half-life values are averages derived from clinical studies and may vary between individuals. Factors that can influence actual half-life include injection site (subcutaneous vs intramuscular absorption rates differ), individual metabolism and body composition, liver and kidney function (which affect clearance), and hydration status. Route of administration also matters significantly — BPC-157 administered orally versus subcutaneously may have very different bioavailability profiles, which effectively changes the functional half-life even if the molecular half-life is the same. Temperature at the injection site can also affect absorption rates.

Dosed includes a built-in half-life calculator that uses published pharmacokinetic data (with PubMed citations where available) to estimate active compound levels over time. After logging a dose, you can see an estimated decay curve showing when levels drop below various thresholds. This helps inform protocol timing decisions and visualize why certain dosing frequencies are used. The app also factors in half-life data when setting up smart reminders — a compound with a 4-hour half-life will have different reminder timing than one with a 7-day half-life. All pharmacokinetic data in Dosed is sourced from published research and clearly cited.