The semaglutide half-life, in short
- ~7 days
- elimination half-life
- ~28 h
- to peak after a single injection
- ~5 weeks
- to reach steady state on a fixed weekly dose
- ~35 days
- to fall below about 3% of a final dose
Five half-lives is the usual rule of thumb for a drug being effectively gone. At seven days each, that is about five weeks after your last injection for semaglutide – longer than most people expect, and considerably longer than tirzepatide, which clears in three to four.
A single dose, week by week
| Days since the shot | Roughly what is left of that dose |
|---|---|
| 1 day | 91% |
| 3 days | 74% |
| 7 days (one half-life) | 50% |
| 14 days (two half-lives) | 25% |
| 21 days | 12.5% |
| 28 days (four half-lives) | 6% |
| 35 days (five half-lives) | 3% |
Decay of a single dose at a seven-day half-life. The first day is a rise rather than a fall, because absorption from the injection site takes about a day.
Why the level doubles before it settles
Read the seven-day row. Exactly half of every dose is still there when the next one arrives. That is the cleanest possible case of accumulation, and it produces a tidy result: at steady state, the level is about twice what a single dose alone would reach.
Getting there takes four to five half-lives, so four to five weeks on a fixed dose. This is why the titration ladder holds every step for four weeks, and why the fourth week of a new dose is often the one that finally tells you what that dose does. Nothing changed; the average was still climbing the whole time.
It also has a practical consequence people rarely have explained to them: judging a dose after ten days is judging something that is still on its way up. The dose schedule guide sets out every ladder with that in mind.
The flatter week
Because the half-life matches the dosing interval, semaglutide loses relatively little between injections compared with tirzepatide. The trough is shallower, and the whole week feels more even.
People moving between the two drugs notice this immediately, usually in one of two ways. From semaglutide to tirzepatide, the week develops a shape: strong for two or three days, then noticeably lighter. From tirzepatide back to semaglutide, the shape flattens out and the “day six” effect disappears. Neither is better; they are different curves. Mounjaro versus Ozempic compares them across the things that actually differ.
A shallower trough is not the same as no trough. Plenty of people on semaglutide still notice appetite creeping back late in the week, and if it happens on the same day repeatedly, that is a real pattern rather than a mood. It is exactly what a level curve laid over a symptom log is for.
What lateness costs on semaglutide
The long half-life is forgiving. A dose taken a day or two late barely registers – you drop a further nine per cent per day rather than falling off anything. Even several days late, the level is still well within the range it has been in all week.
A missed week is more visible, because you spend fourteen days on one dose instead of seven and arrive at roughly a quarter of it rather than a half. Even then the recovery is undramatic: the next dose stacks on what is left and the level climbs back over a couple of weeks. Contrast that with liraglutide, where a missed day is a genuine gap, and it becomes obvious why “what happens if I am late” has no single answer. Missed doses covers each product.
Stopping, and the five weeks afterwards
Stopping semaglutide is slow in both directions. A week after your last injection you still have around half of it. Two weeks, a quarter. A month, about six per cent. The appetite suppression fades over that whole period rather than ending on a particular day, which is why the first fortnight after stopping tells you almost nothing about what life without it will be like.
If you are stopping deliberately – a break, a supply gap, a switch – the log is worth more than usual, because the questions afterwards are all comparative. How fast did the appetite come back. Where did the weight go and when. Which week was hardest. Those are answerable from a record and unanswerable from memory.
Oral semaglutide is the same molecule on a different schedule
Rybelsus and the other tablets have the same seven-day half-life. What changes is the input: a small daily dose instead of one large weekly one, and an absorption step that depends on taking it correctly on an empty stomach. The accumulation arithmetic is the same, so the ladder still needs weeks per step, and the level still climbs to a plateau after each increase. Tracking oral semaglutide covers what is different in practice.
How Penwise models it
Penwise+ uses a one-compartment model with first-order absorption, with the elimination half-life taken from published clinical pharmacology and the absorption rate set so a single dose peaks about 28 hours after injection. Your curve is the sum of the doses you logged, on the dates you logged them.
- It is an estimate from a model, never a measurement of your blood, and never labelled with a clinical unit.
- It is specific to your medication – semaglutide, tirzepatide, retatrutide and liraglutide each have their own parameters.
- It reflects the schedule you actually kept, including late doses and gaps.
- It is never used to suggest a dose.
Stop guessing where you are in the week
Penwise+ estimates your level from your own doses. Fourteen days included, no card and no account.
Common questions
How long does semaglutide stay in your system?
How long does Ozempic take to work?
Is Wegovy the same half-life as Ozempic?
What happens if I take my Ozempic a day or two late?
Why does semaglutide feel steadier across the week than Mounjaro?
Can an app tell me my actual blood level?
Sources
Every factual claim on this page traces to the product information a regulator publishes, or to a published trial. Links open on the site of whoever issued the document.
- Ozempic: EPAR product information European Medicines Agency
- Wegovy: EPAR product information European Medicines Agency
- Rybelsus: EPAR product information European Medicines Agency
Penwise is a tracking and education tool, not a medical device. Nothing on this page is medical advice, a diagnosis or a recommendation to start, stop or change a dose. Dosing decisions belong to you and your clinician. Ozempic, Wegovy, Rybelsus, Saxenda and Victoza are trademarks of Novo Nordisk; Mounjaro, Zepbound and Trulicity are trademarks of Eli Lilly. Penwise is not affiliated with, endorsed by or connected to either company.