GLP-1 Half-Life: How Long Semaglutide and Tirzepatide Stay in Your System
What a half-life actually means for a weekly GLP-1, why levels keep climbing for the first month or two, and why day two and day six of the same week feel so different.
What a half-life actually is
A half-life is the time it takes for half of a drug to leave your system. That is the whole definition, and almost everything confusing about GLP-1 dosing follows from it.
Semaglutide — the active ingredient in Ozempic® and Wegovy® — has a half-life of about a week, roughly 165 hours. Tirzepatide, the active ingredient in Mounjaro® and Zepbound®, is around five days.
Those numbers are why these medications are injected weekly rather than daily. A drug that halves in a week is still substantially present seven days later, which is what makes a once-weekly schedule work at all.
The part that surprises people: levels keep climbing
Here is the consequence that catches nearly everyone out.
If a dose has only half cleared when the next one arrives, the second dose does not start from zero. It stacks on the remainder. So does the third. Each week the leftover from previous doses is larger, and the peak level climbs — even though the dose on the label has not changed.
Working it through roughly, with a one-week half-life and a weekly shot:
| After | Roughly how much of a dose remains |
|---|---|
| 1 week | half |
| 2 weeks | a quarter |
| 3 weeks | an eighth |
| 5 weeks | about 3% |
Add a fresh dose on top of that each week and the total keeps rising until the amount you clear between shots equals the amount each shot adds. That balance point is steady state, and it takes about four to five half-lives to reach — so roughly four to five weeks at an unchanged dose for semaglutide, somewhat less for tirzepatide.
This is why side effects sometimes get worse in week five or six without any dose change, and why a dose that felt fine at first can feel different a month later. Nothing went wrong. The level was still climbing.
It also means the reverse: every time your provider steps you up, the clock restarts and you spend another month or so climbing to a new plateau. On a standard titration schedule you can spend most of your first six months never quite at steady state — which is worth knowing before you conclude that a bad fortnight means the medication is not working. There is more on laying that out in the guide to building a titration schedule.
Why day two and day six feel different
A weekly injection does not give you a flat level for seven days. It gives you a curve.
Levels rise after the shot, peak, then decline across the rest of the week. The peak is not instant — these are slowly absorbed medications, and it typically takes a day or more to get there, which is part of why the worst nausea often lands on day two rather than day one.
Then the level falls all week. By day six or seven you are at the trough — the lowest point of the cycle — right before the next shot puts it back up.
That single fact explains most of what people describe as the rhythm of a week on a GLP-1:
- Days 1–3, near the peak. Side effects cluster here. Nausea, fatigue, reflux, and the strongest appetite suppression.
- Days 4–5, coming down. Usually the most comfortable stretch. Appetite still suppressed, side effects easing.
- Days 6–7, at the trough. Appetite often returns noticeably. Many people describe food noise coming back at the end of the week.
Same dose, same week, different point on the curve. There is a day-by-day version of this in the injection day timeline.
Why this matters for tracking
The practical takeaway is that a symptom entry without a day number is close to useless.
"Felt sick on Tuesday" tells you nothing you can act on. "Felt sick on day two, at 1 mg, in week three of that dose" tells you where you were on the curve and how far into the accumulation you were. The first is a diary. The second is data.
If you record only one extra field alongside your symptoms, make it the day of the shot cycle. It is the field that turns a scattered list into a visible pattern, usually within two or three cycles. The side effect tracking guide covers the rest of what is worth recording.
The same logic applies to weight. A weight taken on day two, when the appetite suppression is strongest and you may be down on fluids, is not comparable to one taken on day seven. Weighing on the same day of the cycle each week removes a source of noise that otherwise looks like progress or a plateau.
What half-life does not tell you
Two cautions, because this is the point where the maths starts to feel more precise than it is.
Your numbers are not the textbook numbers. Published half-lives are population averages. Individual clearance varies with body weight, kidney function, other medications and simple biological variation. A curve calculated from a standard half-life is a useful model of the shape of your week. It is not a measurement of your blood.
The curve does not decide your dose. Understanding why day six feels different is genuinely useful for planning your week and for having a better conversation at your next appointment. It is not a basis for changing your schedule on your own. Dose changes, split dosing and timing changes are decisions for you and your provider together — the arithmetic informs that conversation rather than replacing it.
A note on missed and late doses
Because the half-life is long, being a day late is a smaller event than it feels. The level dips a little further into the trough than usual before the next dose brings it back up.
Being several days late is a different situation, and how far the level has fallen is exactly why the guidance depends on how long it has been. Every GLP-1 comes with specific instructions on missed doses, and they differ between medications — follow the ones that came with yours, and contact your provider if you are unsure rather than working it out from a decay curve.
What you should do in every case is record it. A missed or late dose is the single most useful annotation in a tracking history, because it explains an otherwise inexplicable week when you read the record back two months later.
The short version
- Semaglutide's half-life is about a week; tirzepatide's is about five days.
- Doses stack, so levels climb for roughly four to five weeks after starting or stepping up before they plateau.
- Within each week, levels peak in the first days and trough at the end — which is why side effects and appetite both follow a weekly rhythm.
- Record the day of your cycle alongside anything you track, or the pattern stays invisible.
- The maths describes the shape of your week. Your provider decides your dose.
Ozempic®, Wegovy®, Mounjaro® and Zepbound® are trademarks of their respective owners. This article is general information about how these medications behave, not medical advice about your treatment.