Half-life, explained plainly

What it means

Elimination half-life is the time it takes for the amount of a substance in your body to fall by half. That is the whole definition.

If a compound has a 24-hour half-life and you have 100 units in you now, you have about 50 in 24 hours, 25 in 48, and about 12.5 in 72. The decline is proportional rather than linear — you lose half of whatever remains, so the absolute amount lost gets smaller as levels fall.

Why it sets the dosing interval

A medication with a half-life of a few hours cannot maintain a steady level on weekly dosing; it would be nearly gone within a day. One with a half-life of several days can, because relatively little is lost between injections.

This is the whole explanation for why GLP-1 medications with half-lives measured in days are dosed weekly, while compounds that clear in hours are dosed daily or more often. It is not a matter of potency — it is clearance.

Steady state

When you dose repeatedly, levels do not climb forever. Each dose adds to what remains from previous ones, but the amount eliminated between doses also rises as levels rise. Eventually the two balance, and the pattern from one interval to the next stops changing. That is steady state.

As a rule of thumb, this takes roughly four to five half-lives. For a compound with a seven-day half-life on weekly dosing, that is around four to five weeks — which is a useful thing to know when reading anything about how a protocol is "settling."

The Half-Life Visualizer shows this directly: set a compound and an interval and you can watch the peaks and troughs converge.

Peak and trough

Between doses, levels rise to a peak and fall to a trough just before the next dose. The gap between them depends on the half-life relative to the interval — a long half-life relative to the dosing interval gives a flat curve; a short one gives a pronounced saw-tooth.

This is why the same missed dose has different consequences for different compounds. Miss a dose of something that clears slowly and the curve dips modestly. Miss one that clears quickly and it falls a long way.

The three caveats that matter

These are the reasons a curve on a screen is not a fact about your body.

• An estimated curve is a model, not a measurement. It shows the shape implied by a published half-life figure and your inputs. Only a blood test measures what is actually in you.
• Published half-lives are population averages. Individual clearance varies, sometimes substantially. A figure quoted as "about 7 days" describes a distribution, not everyone.
• Depot injections are release-limited, not elimination-limited. For an oil-based ester, the long duration comes from slow release out of the injection site, not from the hormone itself clearing slowly. The figure quoted is an apparent half-life for the whole system.

That last point matters when reading our figures for testosterone and estradiol esters — the notes on each entry in the Half-Life Data Table say which kind of number it is, and every entry carries an evidence tag showing whether it comes from an FDA label, a clinical trial, a published PK study, or is not well characterised.

What half-life does not tell you

It describes how a substance leaves your system. It says nothing about how well something works, how you will feel, what dose is appropriate, or how long an effect lasts — effects and concentrations are related but not the same thing. Read How the Estimate Works for what our model does and does not claim.

Frequently asked