What half-life is, why it varies between esters and molecules, and how it determines how often you need to inject.
Half-life is the most important number on the DoseCurve chart. Change it and the entire curve changes shape. This page explains what it actually means, why different esters have wildly different values, and how to use it to reason about dose frequency.
A compound's half-life is the time it takes for half of what's currently in your system to be cleared. That's it. If you have 200 mg in your system and the half-life is 7 days, then in 7 days you'll have 100 mg. In 14 days, 50 mg. In 21 days, 25 mg.
Note what the definition does not say: it does not say "the time until the drug is gone." Half-life is a fractional measurement — there's always half of what was there a half-life ago.
After five half-lives, roughly 97% of a single dose has cleared. That's the practical definition of "out of your system." For testosterone cypionate (half-life ~8 days), one dose is essentially gone after about 40 days. For a fast peptide with a half-life of 30 minutes, it's gone in about 2.5 hours.
The same rule applies in reverse: when you start a new protocol, it takes about five half-lives of repeated dosing to reach steady state — the point where the dose going in equals the dose being cleared each cycle. More on that in Steady state.
Pure testosterone has a half-life of minutes. You couldn't realistically inject it. The trick of esterification is to attach a fatty acid chain that makes the molecule oil-soluble, so it forms a depot at the injection site and is released slowly into circulation. The longer the chain, the slower the release.
Approximate half-lives for common testosterone esters:
| Ester | Approx. half-life | Typical frequency |
|---|---|---|
| Propionate | ~1–2 days | Every other day |
| Phenylpropionate | ~4 days | Twice weekly |
| Enanthate | ~7 days | Weekly or twice weekly |
| Cypionate | ~8 days | Weekly or twice weekly |
| Undecanoate (oil) | ~20–30 days | Every 10–12 weeks |
These are population averages. Your individual half-life can differ by ±30% based on body composition, metabolism, and injection site.
Two protocols with the same total weekly dose can produce very different curves. 200 mg once a week and 100 mg twice a week deliver the same average — but the twice-weekly version has roughly half the peak-to-trough swing. Frequency is the lever for stability; total dose is the lever for average level.
Try it on the chart: enter the same weekly total with different frequencies and watch the shape change. The line gets flatter as frequency increases, asymptotically approaching the curve of a continuous infusion.
The substance presets on DoseCurve are population-average starting points. If bloodwork from a clinician-supervised protocol consistently disagrees with the modelled curve, the assumed half-life may not match the individual. Discussing the mismatch with the prescribing clinician — and, if appropriate, editing the half-life in the calculator — can make the visualisation a more useful conversation aid.
The model is a teaching aid, not a calibration tool. Bloodwork ordered and interpreted by a clinician remains the ground truth.
DoseCurve is an educational tool. Nothing on this site is medical advice. Always consult a qualified clinician before starting, changing or stopping any protocol.