“Half-life” shows up in every conversation about GLP-1 medications, often without an explanation. Here is what it means for semaglutide, the medication in Wegovy, Ozempic and Rybelsus.
What half-life means
The half-life of a medication is the time it takes for the amount in your body to drop by half. For semaglutide, that is about 7 days.
So if you take one dose and nothing else:
| Time after the dose | Share still in your body (roughly) |
|---|---|
| 1 week | 1/2 |
| 2 weeks | 1/4 |
| 3 weeks | 1/8 |
| 4 weeks | 1/16 |
| 5 weeks | 1/32 |
This long half-life is the reason semaglutide can be a once-a-week shot. Most older GLP-1 medications cleared much faster and had to be taken daily.
Why the level builds up over weeks
Because half of each dose is still there a week later, every new shot adds to what is left. The level climbs over the first weeks on a dose, then settles into a steady weekly rhythm. According to the prescribing information, that steady state is reached after 4 to 5 weeks of weekly dosing.
This also explains why the first weeks on a new dose can feel different from later ones: the level is still building.
What happens within a week
After an injection, semaglutide is absorbed slowly and peaks 1 to 3 days later. It then eases off until the next shot. Once settled, the day before your next shot is the lowest point of your week, which some people notice as appetite coming back. We cover that in why hunger comes back before shot day.
How long it takes to clear
Pharmacology uses a rule of thumb: after about 5 half-lives, most of a medication is gone. For semaglutide, that is about 5 weeks after the last dose. It is a rule of thumb, not an exact date, and it varies between people.
What half-life does not tell you
- It does not tell you what dose to take or when. Your prescriber decides that.
- It does not describe your personal level: body weight, metabolism and other factors change it.
- It does not tell you what to do after a missed dose. The instructions that came with your medication cover that.
See it on a chart
The GLP-1 level estimator draws the week after your shot, using the published half-life. Choose Wegovy or Ozempic and see how the curve changes in your first weeks on a dose.