What it means
After a dose, the level of a drug in the blood rises, then falls as the body clears it. The half-life is how long it takes to drop by half. After about four to five half-lives, most of a drug is gone. It takes about the same time to reach a steady level when you take a drug regularly.
Half-lives explain GLP-1 dosing. Exenatide lasts a few hours, so the original version was given twice a day. Liraglutide's half-life is about 13 hours, so it is daily. Orforglipron's is roughly 29 to 49 hours, also daily. Tirzepatide's is about 5 days and semaglutide's about a week, so both are weekly.
A long half-life also means a drug lingers after you stop. That is why the Wegovy label advises stopping at least 2 months before a planned pregnancy, and why missed-dose windows differ between medicines.
Why it matters
Half-life explains dosing frequency, missed-dose rules and how long effects and side effects last after stopping.
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