How long does it take to work?
When Mounjaro, Wegovy, Ozempic or Saxenda starts working, and how long it stays in your system after your last dose. Pick your medicine for a timeline from the leaflet, plus a calculator for when it’s cleared.
Updated 31 October 2026
- Taken
- once a week
- Half-life
- about 5 days
- Out of your system
- about 3 to 4 weeks
When Mounjaro starts working
Many people notice less appetite within the first week or two. How fast you lose weight varies a lot from person to person.
Day 1
Your first injection (2.5 mg)Leaflet
You start low, at 2.5 mg, to let your stomach settle. This isn’t the dose that drives the most weight loss yet.
From week 1
Appetite starts to dropLeaflet
The leaflet says tirzepatide lowers hunger and how much you eat from the first week of treatment.
From week 3
You feel fuller for longerLeaflet
Increased fullness after eating is described from week 3.
Week 5
First full dose (5 mg)Leaflet
After 4 weeks at 2.5 mg, the dose goes up to 5 mg, the first dose meant for a lasting effect. It can rise to 10 or 15 mg later, at least 4 weeks apart.
Over the months
The full effect buildsTrial
Weight loss begins in the early weeks but keeps going for months as the dose steps up. In the main trial the average loss at 72 weeks on 15 mg was about 21%.
Still in your system?
When was your last Mounjaro dose? We’ll work out roughly when it’s essentially cleared.
Having an operation? If you’re having an operation or an endoscopy, tell your surgical team you’re on it. It slows how fast your stomach empties, which matters for anaesthetic, and they’ll tell you whether to pause it first.
Planning a pregnancy? If you’re planning a pregnancy, ask your prescriber how long to stop beforehand. These can take a while to leave your body, so the advice is usually to stop ahead of time.
These medicines aren’t checked for on standard drug tests.
What happens if you stop
Appetite tends to return and weight tends to come back after stopping, so it’s used as a long-term treatment, not a short course.
General information from the UK patient leaflets and trial results, not medical advice, and not a prediction for you. Everyone responds differently. Don’t start, stop or change a dose without talking to your prescriber.
Common questions
- How long does Mounjaro take to work?
- The leaflet says tirzepatide lowers hunger from the first week and increases fullness from week 3. You start at 2.5 mg and reach the first full dose (5 mg) at week 5, so the effect builds over the first weeks and months as the dose steps up. How fast you lose weight varies a lot from person to person.
- How long does Wegovy take to work?
- The leaflet says weight loss started early and continued throughout the trial. Wegovy has a fixed 16-week step-up to the 2.4 mg maintenance dose, and the steepest weight loss usually comes once you’re on it, so the full effect takes many months.
- How long does it stay in your system after the last dose?
- For semaglutide the leaflets give a figure: Wegovy (2.4 mg) stays in the blood for about 7 weeks after the last dose, and Ozempic for about 5 weeks. The leaflets don’t state one for Mounjaro or Saxenda, so from their half-lives (about 5 days and about 13 hours) they’re essentially gone after about 3 to 4 weeks and about 3 days. Use the calculator above for a date from your last dose.
- Do I need to stop it before surgery?
- These medicines slow how fast your stomach empties, which matters for an anaesthetic. If you’re having an operation or an endoscopy, tell your surgical team you’re on it and they’ll tell you whether to pause it first. Don’t stop or change anything without asking.
- Will it show up on a drug test?
- No. Mounjaro, Wegovy, Ozempic and Saxenda aren’t checked for on standard drug tests. The reason clearance usually matters is surgery or planning a pregnancy, not a drug screen.
- What happens if I stop taking it?
- Appetite tends to return and weight tends to come back after stopping, which is why these are used as long-term treatments rather than short courses. In Wegovy’s own withdrawal trial, people who switched to a dummy injection regained weight while those who stayed on the full dose kept losing. Talk to your prescriber before stopping.
Half-lives, clearance and onset from the UK patient leaflets and SmPCs for Mounjaro, Wegovy, Ozempic and Saxenda, with weight-loss figures from their pivotal trials.