What is Half-Life?
Half-life (t½) is the time it takes for the concentration of a substance in the bloodstream to fall to half its initial value. It's a fundamental pharmacokinetic parameter that governs dosing frequency, peak and trough levels, and how long a compound remains active in the body.
For peptides, half-life varies enormously — from minutes (native GHRH: ~5 min) to days (CJC-1295 with DAC: 6–8 days). Understanding half-life is essential for timing injections correctly and maintaining stable serum levels.
First-Order Elimination
Most peptides follow first-order kinetics — the rate of elimination is proportional to the current concentration. This means the same fraction (not amount) is eliminated per unit time. Mathematically:
C(t) = C₀ × e^(-kt)
Where C₀ is the initial concentration, k is the elimination rate constant (k = ln(2) / t½), and t is time. This produces the characteristic exponential decay curve.
Accumulation and Steady State
When you dose repeatedly, each new dose adds to the remaining concentration from previous doses. After approximately 4–5 half-lives of repeated dosing, serum levels reach steady state — where the amount eliminated between doses equals the amount added by each new dose.
For example, with Ipamorelin (t½ ≈ 2 hours), steady state is reached within 8–10 hours of starting 3× daily dosing. With CJC-1295 DAC (t½ ≈ 168 hours), it takes about 4–5 weeks of weekly injections to reach true steady state.
Superposition Principle
Because first-order kinetics are linear, the total serum concentration at any point is simply the sum of the contributions from each individual dose, each undergoing its own exponential decay from the time it was administered. This is called the superposition principle, and it's exactly how Peptalytix models your serum levels:
C_total(t) = Σ C₀ × e^(-k × (t - t_dose))
The graph shows each peptide's current estimated serum level and the combined total, updated in real time based on your logged injections.
Why Timing Matters
Different contexts require different concentration profiles:
- GH secretagogues: Pulsatile release is important — mimicking the body's natural GH pulse rhythm. Dosing 2–3× daily on an empty stomach creates distinct peaks rather than a sustained baseline, which may better preserve pituitary sensitivity.
- BPC-157: With a ~4 hour half-life, once or twice daily dosing maintains meaningful serum levels throughout most of the day.
- Semaglutide: Weekly dosing with its ~7 day half-life maintains a very stable, smooth serum level — minimising peaks and troughs that could cause nausea or inconsistent appetite suppression.
The Trough Problem
The trough is the lowest serum level, occurring just before the next dose. If troughs fall too low, you lose coverage — periods where the peptide is essentially inactive. If peaks are too high relative to troughs, you may experience side effects at peak and lack efficacy at trough.
The peak-to-trough ratio is controlled by dosing frequency relative to half-life. Dosing every half-life (e.g., Ipamorelin every 2 hours) would give a peak:trough ratio of 2:1. Dosing every 5 half-lives would give a ratio of ~33:1. Most protocols target a ratio that balances efficacy against injection burden.
Using Serum Level Graphs
Peptalytix's PK graph uses your actual injection history — the exact dose and time of each logged injection — to compute your real-time serum level estimates. This lets you:
- Visually confirm that your dosing schedule maintains desired levels
- Identify gaps in coverage (missed doses or troughs dropping too low)
- Understand how long after stopping a peptide it will be cleared
- Compare relative contributions of multiple peptides to total serum load
Keep in mind that half-life values for research peptides are estimates based on limited pharmacokinetic data — individual variation (body weight, kidney function, injection site, and metabolism) can significantly affect actual clearance rates.