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How half-life curves work
Every dose curve on your dashboard plots active amount over time — not just dose timing. This page explains where the numbers come from.
The basic idea
When you take a compound, two things happen:
- Absorption — the compound enters your bloodstream from wherever you administered it (subcutaneous tissue, an oral mucosa, an IV).
- Elimination — your body clears it (liver, kidneys, breakdown).
Both processes follow first-order kinetics in the simple case: at any moment, the rate of change is proportional to how much is left. That gives you exponential curves and a single number — the half-life — that summarizes them.
Half-life
The half-life is how long it takes for the amount of a compound in your system to drop to half of what it was. It's a property of the compound (and you, but the variation is small relative to the average).
Some half-lives the app knows:
| Compound | Half-life |
|---|---|
| Tirzepatide | ~5 days |
| Semaglutide | ~7 days |
| Liraglutide | ~13 hours |
| Dulaglutide | ~5 days |
| Retatrutide | ~6 days |
A 5-day half-life means: after 5 days, half the dose remains active. After 10 days, a quarter. After 15, an eighth. And so on.
The math
For a single dose of a compound with elimination rate $k_e = \ln(2) / t_{1/2}$, the active amount $A(t)$ at time $t$ after dosing is:
Bolus (instant peak): $$A(t) = D \cdot e^{-k_e t}$$
Where $D$ is the dose. This is what you'd see if you took the compound IV — instant peak, exponential decay.
Sub-Q or oral (Bateman absorption + elimination):
Real-world peptides aren't bolus. They absorb through tissue over hours, peak, then eliminate. That's the Bateman function:
$$A(t) = D \cdot \frac{k_a}{k_a - k_e} \cdot (e^{-k_e t} - e^{-k_a t})$$
Where $k_a = \ln(2) / t_{1/2,abs}$ is the absorption rate.
For the app's defaults:
- Sub-Q absorption half-life: ~6 hours (0.25 days).
- Depot absorption half-life: ~24 hours (1 day).
The Bateman curve rises from zero to a peak, then declines. Sub-Q peaks fast, depot peaks slow and lower.
What you see on the dashboard
For each enabled compound, the dashboard plots one curve. Each curve is the sum of all your dose contributions: at any moment, the line shows the total active amount across all doses you've taken.
This is what makes weekly compounds interesting. With a 5-day half-life and a weekly schedule, by the time the next dose lands, you still have most of the previous one. You accumulate to a steady state over several weeks — the floor (trough before next dose) and ceiling (peak after) both rise until they level off.
Stacked doses
If you take more than one compound, each gets its own curve. They don't interact in the math — each curve is independent. See How stacked doses combine for the longer version.
If two compounds act on the same receptor (Tirzepatide + Retatrutide both hit GLP-1R, GIPR), the clinical effect combines, but the active-amount curves are still plotted separately. Combining receptor occupancy is what the endogenous biomarker simulation does.
What "active amount" actually means
The active-amount curve plots the mass of compound remaining (in mg, mcg, iu, or whatever unit the compound uses), not its effect.
Effect doesn't track active amount linearly. GLP-1 receptor agonists, for example, saturate — past a certain receptor occupancy, more compound doesn't add more effect. The curve tells you how much is in your system. Effect modeling is the job of the simulation.
Caveats
- Individual variation matters. Half-lives in the table above are population averages. Your liver, kidneys, body composition, age, and concomitant drugs can shift them by ±20% or more. The curve is an estimate, not a measurement.
- Custom compounds inherit the kinetics shape you pick. If you set up a custom peptide as "sub-Q" but it's actually a long-acting depot formulation, the curve shape will be wrong. See Kinetics shapes.
- The app doesn't model bioavailability. Oral compounds have low bioavailability (Rybelsus is ~1%). The catalog adjusts the effective dose where applicable; for custom compounds, you'd enter the effective dose, not the swallowed dose.
- Half-life isn't constant for everyone. Compromised liver/kidney function lengthens it. The numbers above assume healthy adults.
Where to change kinetics for custom compounds
Profile → Compounds → [your compound] → Half-life / Kinetics shape.
For built-in GLP-1s, kinetics are fixed by the canonical catalog and aren't editable. If you have reason to believe yours metabolize differently (which would be unusual), use a custom compound entry instead.
Related
- Kinetics shapes — bolus, sub-Q, depot.
- How stacked doses combine — multi-dose math.
- Endogenous biomarker simulation — what the app does with the curves.