One injection, seven days of effect. If most of your experience with medication involves pills that work for a few hours and are gone by bedtime, weekly semaglutide (the molecule in Ozempic and Wegovy) can feel like a small mystery. How does a Tuesday shot still matter on Sunday? Why does the first month feel like a slow ramp? And why does being a day late not undo weeks of progress?
All three questions have the same answer, and it is a single number: semaglutide's half-life of roughly one week. Understand that number and the whole weekly rhythm snaps into focus. No chemistry degree required.
What "half-life" actually means
A medication's half-life is the time your body takes to clear half of whatever amount is currently circulating. The key word is half. Not all of it: half of it, over and over again.
Per the prescribing information, semaglutide's half-life is approximately one week, around 165 hours. So if you stopped injecting today, about half of the medication would still be in your system a week from now. Two weeks out, about a quarter would remain. Three weeks out, roughly an eighth. The decline is a long, gentle slope, not a switch flipping off.
| Time after a final dose | Roughly how much remains |
|---|---|
| 1 week | About half |
| 2 weeks | About a quarter |
| 3 weeks | About an eighth |
| 4 weeks | About a sixteenth |
Two things are worth noticing here. First, the shot you took three weeks ago is still, faintly, on the field. Second, these are averages from published pharmacology; individual bodies clear medication at somewhat different rates, which is one of several reasons this article describes a model, not your bloodstream.
Why a weekly shot works at all
Because so little clears in seven days, each new injection does not start from zero. It lands on top of medication still left over from the previous several weeks. Your shot this Tuesday is topping up a reservoir that is still more than half full from everything that came before.
Compare that with shorter-acting relatives in the same family. Liraglutide (Saxenda, Victoza) has a half-life of about 13 hours, which is why it is a daily injection. Exenatide (Byetta), the original GLP-1 medication, lasts only a few hours and is taken twice a day. Semaglutide's long half-life is precisely what makes a once-weekly schedule possible: the medication itself bridges the gap between shots.
The first month: filling the reservoir
Picture a bathtub with the drain partly open. Once a week you pour in a bucket of water; all week long, the drain steadily lets a fraction out. In the early weeks, you are pouring in more than the drain removes, so the water level climbs. But the drain removes a percentage of whatever is in the tub, so as the level rises, so does the outflow. Eventually the weekly bucket and the weekly drainage balance out, and the level settles into a steady rhythm: a small rise on shot day, a gentle ebb through the week, around a stable average.
Pharmacologists call that plateau steady state, and a common rule of thumb is that it takes about four to five half-lives to get there. For semaglutide, that is about a month. At a constant dose, your level in week four is meaningfully higher than in week one, even though every injection was identical.
This explains a lot of early GLP-1 experience. It is part of why the first weeks on a new dose can feel underwhelming, and why the same dose can feel different in week four than it did in week one. It also fits the design of the dose ladders: Ozempic and Wegovy titration steps are typically four weeks long (and the 0.25 mg starting dose is, per the label, a starting dose rather than a maintenance dose). Each step gives the level time to settle near its new plateau, and gives your digestive system time to adapt, before you and your prescriber judge how it is going. Our guide to GLP-1 titration schedules walks the ladders in detail.
A missed dose is a dip, not a cliff
Now run the same math on a mistake. Suppose your shot day comes and goes and you only notice two days later. In those two days, well under half of your medication has cleared; the level has eased modestly, nothing more. You are standing on a dip in the curve, not at the bottom of a cliff.
The official guidance reflects exactly this. The Ozempic label allows a missed dose to be taken within 5 days; Wegovy says take it as soon as possible if your next scheduled dose is more than 2 days away, and skip it otherwise. And every label agrees on one rule: never take a double dose to make up for a missed one. Our missed dose guide summarizes the windows for every GLP-1 medication.
After a late or skipped dose, the level dips and then recovers over the following weeks as regular dosing resumes. The same slow math that makes the first month gradual also makes the system forgiving: one imperfect week rarely matters much, as long as it stays one.
GLP 1 Tracker AppFree on the App Store. Log every dose and watch an educational level curve built from your own shots.
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What the app's level curve shows
GLP 1 Tracker App turns this math into a picture. From the doses you log, it draws an educational medication level curve using published half-life values like the ones above: the line rises after each logged shot and eases off between them. On it you can see the first-month climb, the weekly ripple around steady state, and the dip left by a late dose, all built from your own timestamps rather than a textbook's idealized patient.
It is worth stating plainly what that curve is and is not. It is an estimate for education, not a measurement. No app can know your actual blood concentration; absorption, body composition and clearance vary from person to person, and the only way to measure a real level is a laboratory test. The curve applies published averages to your dose history so the shape of the pharmacology becomes visible. It never tells you what to take or when: the app does not advise doses, and those decisions stay between you and your prescriber.
What the curve cannot tell you
- Your actual blood level. That requires a lab, not an app.
- How you will feel on a given day. Appetite and side effects do not map neatly onto the line.
- Whether to take a late dose. That answer comes from the label and your prescriber, not from a chart.
- Whether your dose is right. Only your prescriber, looking at the whole picture, can judge that.
A fair way to hold it: the curve is a diagram of the system, not a sensor inside you. Used that way, it is one of the most clarifying pictures in GLP-1 life, because it replaces vague worry ("is it even in my system anymore?") with a calm, roughly right shape.
The takeaway
Semaglutide's half-life is about a week. That single fact explains the weekly schedule, the month-long climb to steady state, the four-week titration rhythm, and the forgiving missed-dose windows. The math only becomes useful when it runs on real timestamps, though, which is the quiet argument for logging every shot the moment it happens. If you are on Ozempic, our Ozempic tracking guide covers the full weekly routine. And if your medication is Mounjaro or Zepbound, the same logic runs on a faster clock: see tirzepatide's five-day half-life.
Frequently asked questions
How long does semaglutide stay in your system?
Semaglutide's half-life is about one week, roughly 165 hours, per the prescribing information. After a final dose, about half remains a week later, a quarter after two weeks, and an eighth after three. Meaningful amounts take a month or more to fade. These are published averages; individual bodies clear medication at somewhat different rates, so only a laboratory test can measure an actual level.
How long does it take semaglutide to reach steady state?
About a month at a constant dose. A common rule of thumb is that a medication reaches steady state after four to five half-lives, and semaglutide's half-life is about one week. Each weekly shot lands on medication left over from earlier weeks, so the level climbs at first and then settles into a stable weekly rhythm. That is part of why week four on a dose can feel different from week one.
Can I take my Ozempic or Wegovy shot a day late?
A short delay leaves you on a dip in the level, not a cliff, because well under half of the medication clears in a day or two. Per the labels, a missed Ozempic dose can be taken within 5 days; Wegovy says take it as soon as possible if the next scheduled dose is more than 2 days away, and skip it otherwise. Never take a double dose. When in doubt, ask your prescriber.
Can an app tell me my actual semaglutide blood level?
No. GLP 1 Tracker App draws an educational medication level curve from the doses you log, using published half-life values, and that curve is an estimate for education, never a measurement. Absorption, body composition, and clearance vary from person to person, and only a laboratory test measures a real level. The curve shows the shape of the pharmacology; it never advises doses.
Why does a weekly semaglutide shot keep working all week?
Because so little clears in seven days. Each injection lands on top of medication still remaining from the previous several weeks, so the level eases only gently between shots instead of dropping to zero. That long half-life is what makes a once-weekly schedule possible, and it is why keeping the same shot day keeps the level steady.