Definition: Half-life is the time it takes for the amount of a drug in your body to fall by half. After one half-life, 50% of a dose is left; after two, 25%. Most drugs follow this pattern, called first-order elimination, so the same half-life applies whether the level is high or low.
Why it matters when you track doses
Half-life sets three things you will notice: how often a drug is dosed, how long it takes to settle into a steady pattern, and how long it lingers after you stop. A drug with a long half-life can be dosed less often and is more forgiving of a late dose. A drug with a short half-life drops faster between doses.
Some label figures: the Ozempic and Wegovy labels give semaglutide an elimination half-life of about 1 week. The Mounjaro label gives tirzepatide about 5 days. The Depo-Testosterone label gives testosterone cypionate about 8 days when injected into muscle.
Worked example: a 1-week half-life
How much of one semaglutide dose is left over time
- Start with 100% of a single dose (ignoring the first few days while it is absorbed).
- After 1 week: 100% × 0.5 = 50%.
- After 2 weeks: 50% × 0.5 = 25%. After 3 weeks: 12.5%. After 4 weeks: 6.25%.
- After 5 weeks: 6.25% × 0.5 = about 3%. This matches the Ozempic label, which says semaglutide stays in circulation for about 5 weeks after the last dose.
- For tirzepatide (about 5 days), 5 half-lives is 5 × 5 = 25 days.
This uses the simple model where each half-life halves what is left. Real people vary with body size, injection site and kidney or liver function.
You can run your own numbers with the half-life calculator.
Common mix-ups
- Half-life is not how long the effect lasts. Effects can fade before or after the drug is gone.
- Half-life in the body is not shelf life in the vial. A long half-life says nothing about how long a mixed vial stays good.
- Testosterone esters: the ester slows release from the injection site. The enanthate label puts free testosterone itself at 10 to 100 minutes; the days-long figure belongs to the injected ester.
Read more on Dose Track: Half-life calculator · Missed dose guides · Semaglutide guide
General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.
How many half-lives until a drug is gone?+
After 5 half-lives about 3% of a single dose is left, so it is often treated as mostly cleared. It is never exactly zero in the simple model, but it becomes too small to matter for most purposes.
What is the half-life of semaglutide?+
About 1 week, according to the Ozempic and Wegovy labels. The labels say it stays in circulation for about 5 weeks after the last dose.
Does a bigger dose have a longer half-life?+
For most drugs, no. With first-order elimination the half-life is the same at any level; a bigger dose just starts from a higher point.