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Pharmacokinetic tool

Half-Life & Dosing Interval Calculator

Simulate the steady-state concentration curve for any research peptide, with a clear view of the accumulation factor that drives interval selection.

Inputs

mcg

Half-life

168 h

Time to steady

~35 d

Accumulation

×2.00

Avg SS level

1.00 mg

Steady-state concentration curve

Single-compartment first-order elimination model. Each spike represents an administration; the decay between is exponential with the peptide's half-life.

Steady-state plasma concentration over 28 days for Semaglutide at 500 mcg every 168 hours. Accumulation factor 2.00; average steady-state 1.00 mg.2344697039377d14d21d28dmcg

The arithmetic

The simulator uses a single-compartment first-order elimination model. The elimination rate constant k is derived from the peptide's half-life: k = ln(2) ÷ t½. The accumulation factor at a given interval τ is 1 ÷ (1 − e−kτ) — the multiplier applied to a single-dose concentration to obtain the steady-state average. Practical steady state is generally reached after five half-lives.

Why interval matters

A peptide with a 168-hour half-life such as Semaglutide reaches steady state in roughly 35 days, and its accumulation factor on a weekly schedule is around 2.0 — meaning the average steady-state concentration is double the single-dose peak. By contrast, a peptide with a 30-minute half-life such as Tesamorelinachieves no measurable accumulation; each dose acts independently.

Supporting research

  • Knudsen LB et al., Mechanism of GLP-1 receptor agonist pharmacokinetics, Diabetes Obes Metab, 2010.
  • Falutz J et al., Long-term safety and effects of tesamorelin, Clin Endocrinol, 2010.
  • Rowland M, Tozer TN, Clinical Pharmacokinetics and Pharmacodynamics (4th ed), Lippincott, 2011.

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Frequently asked questions

What is a peptide half-life?
Half-life (t½) is the time required for plasma concentration of a peptide to fall by half. It is a property of the molecule, not the dose — doubling the dose doubles the peak, but the time to halve that peak is unchanged.
How long does it take for a peptide to reach steady state?
Practical steady state is reached after roughly five half-lives. For Semaglutide (t½ ≈ 168 hours) that is about 35 days; for Tesamorelin (t½ ≈ 0.4 hours) it is under three hours, meaning each dose acts essentially independently.
What is the accumulation factor?
The accumulation factor at a given interval τ is 1 ÷ (1 − e^(−kτ)), where k = ln(2) ÷ t½. It is the multiplier applied to a single-dose concentration to obtain the steady-state average — for Semaglutide on a weekly schedule it is about 2.0.
Why does the calculator show spikes in the concentration curve?
Each spike represents an administration. The decay between spikes is exponential at the peptide's half-life. Over time the spikes flatten because they sit on top of accumulating residual concentration.
Does the calculator model multi-compartment kinetics?
No — it uses a single-compartment first-order elimination model. That is accurate for most subcutaneously administered research peptides; multi-compartment effects are negligible at the timescales of weekly or daily dosing.