At a glance
Both are prescription-only weekly injections licensed in the UK for weight management, used alongside a reduced-calorie diet and more physical activity. Both start at a low dose and step up roughly every four weeks.
Mounjaro and Wegovy compared | Mounjaro | Wegovy |
|---|
| Active ingredient | Tirzepatide | Semaglutide |
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| Hormones it acts on | GIP and GLP-1 | GLP-1 |
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| How it's taken | Injection, once a week | Injection, once a week |
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| Starting dose | 2.5 mg | 0.25 mg |
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| Highest dose | 15 mg | 2.4 mg |
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| Dose steps | Every 4 weeks, as tolerated | Every 4 weeks, as tolerated |
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| Average weight loss vs placebo | About 20% vs 3% (SURMOUNT-1) | About 15% vs 2% (STEP 1) |
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| Head-to-head (SURMOUNT-5) | 20.2% after 72 weeks | 13.7% after 72 weeks |
|---|
| Common side effects | Nausea, diarrhoea, constipation, indigestion | Nausea, diarrhoea, constipation, headache |
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| Price per pen | £194.99 to £349.99 | £144.99 to £289.99 |
|---|
How they work
Wegovy contains semaglutide, which acts like GLP-1, a hormone your gut releases after eating. It reduces appetite, slows how quickly the stomach empties and helps you feel full for longer.
Mounjaro contains tirzepatide, which acts on GLP-1 and a second hormone, GIP. Acting on both is thought to explain why average weight loss in trials has been larger.
What the trials found
In their main trials against placebo, people on the highest dose of Mounjaro lost about 20% of their body weight on average over 72 weeks, and people on Wegovy about 15% over 68 weeks. Those trials were run separately, in different people, so the figures aren't a direct comparison.
SURMOUNT-5 compared the two directly in adults with obesity but without diabetes. After 72 weeks, average weight loss was 20.2% with tirzepatide and 13.7% with semaglutide. Averages hide wide variation: some people lose much more, some much less, and how well you tolerate a medicine matters as much as the average.
Side effects
The side effects are similar because both act on GLP-1: nausea, diarrhoea, constipation and indigestion, mostly in the first weeks and after each dose increase. Rare but serious risks, including pancreatitis and gallbladder problems, apply to both. Our side effects guide covers what's normal and when to get help.
Cost
Both are priced per pen, and each pen lasts four weeks. The price rises with the dose, so the first months cost less than the later ones. Every dose is priced on the Mounjaro and Wegovy pages, and side by side on our pricing page.
How the choice is made
There isn't one right answer for everyone. A prescriber decides which medicine, if either, is suitable for you, based on your BMI and health, other medicines you take, how you've got on with any weight-loss medicine before, and what matters to you, such as cost.
Our online service treats adults aged 18 to 65 with a BMI of 27 to 50. Between 27 and 29.9 it usually recommends Wegovy; from 30, both are options. Who can be treated explains the full criteria.
Switching between them
People do switch, usually because of side effects, cost, or results that have stalled. The doses aren't equivalent, so you don't simply carry your dose across: a prescriber decides the starting dose on the new medicine, and you shouldn't take both. Tell your prescriber about any weight-loss medicine you've taken in the past few months.
If you'd prefer a tablet
The Wegovy Pill contains the same medicine as Wegovy, semaglutide, as a daily tablet taken on an empty stomach. It starts from £138.99 per pack.
Sources
- Aronne LJ et al. Tirzepatide as Compared with Semaglutide for the Treatment of Obesity (SURMOUNT-5). N Engl J Med 2025
- Jastreboff AM et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). N Engl J Med 2022
- Wilding JPH et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med 2021
- NICE TA1026: Tirzepatide for managing overweight and obesity
- NICE TA875: Semaglutide for managing overweight and obesity
General information only. A prescriber decides which treatment, if any, is suitable for you.
