Half-Life
Also known as: t½ · elimination half-life · biological half-life
The time required for the concentration of a peptide in the body to fall to half of its peak value.
Half-life is the single most load-bearing pharmacokinetic variable in peptide protocol design. It dictates how often a compound must be redosed to maintain steady-state plasma concentrations, how long it takes to clear after discontinuation, and how much overlap to expect between injections. A short half-life compound (e.g., BPC-157 at roughly 4 hours systemic) requires multiple daily injections to sustain tissue exposure, whereas a long half-life compound (e.g., Semaglutide at ~165 hours) supports weekly dosing.
In practitioner audits we've reviewed, dosing protocols that ignore half-life math routinely produce trough concentrations that fall below therapeutic thresholds — explaining why some researchers report inconsistent results with identical milligram doses. DoseCraft's PK-aware copilot models half-life decay curves per compound so researchers can see tissue exposure between injections rather than guessing from static dose tables.
Tirzepatide's half-life of ~120 hours supports once-weekly injection; the plasma concentration at day 7 is roughly 49% of peak, which is still above the threshold for sustained GLP-1/GIP receptor engagement.
Related Terms
Cmax
The maximum plasma concentration reached after a single dose of a peptide.
Cmin
The lowest plasma concentration of a peptide during a dosing interval, typically measured immediately before the next dose.
Steady State
The point at which the rate of peptide administration equals the rate of elimination, producing a stable average plasma concentration.
AUC (Area Under Curve)
The integrated total exposure of the body to a peptide over time, calculated as the area under the concentration-time curve.
Clearance Rate
The volume of plasma cleared of a peptide per unit time, typically by renal filtration, hepatic metabolism, or proteolytic degradation.
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