Mounjaro and Wegovy are the two leading prescription weight-loss injections in the UK. Both are once-weekly, both are MHRA-licensed medicines, and both produce clinically significant weight loss — but they contain different active ingredients that work in slightly different ways. In head-to-head trial data, Mounjaro (tirzepatide) has produced greater average weight loss, while Wegovy (semaglutide) carries a UK licence for reducing the risk of serious cardiovascular events in eligible patients — a benefit Mounjaro doesn't yet hold.
The right choice depends on your health history, goals and how you respond — which is why a prescriber makes the final call. Here's what you need to know to have that conversation well-informed.
The key difference: one hormone pathway vs two
Wegovy contains semaglutide, which mimics a natural gut hormone called GLP-1. GLP-1 slows digestion, reduces appetite and helps regulate blood sugar. Mounjaro contains tirzepatide, which activates the GLP-1 receptor and a second receptor called GIP. This dual action appears to produce a stronger effect on appetite and metabolism, which is reflected in trial results.
How do the results compare?
In their pivotal trials, participants taking semaglutide 2.4 mg lost around 15% of their body weight over 68 weeks (STEP 1), while participants taking tirzepatide lost up to around 21% over 72 weeks at the highest dose (SURMOUNT-1). In the first large head-to-head trial comparing the two directly in people with obesity (SURMOUNT-5), tirzepatide produced greater average weight loss than semaglutide over 72 weeks.
Averages hide individual variation, though. Some people respond better to semaglutide; some tolerate one far better than the other. Trial numbers are a guide, not a guarantee.
Where Wegovy stands out: heart health
In the SELECT trial of over 17,000 adults with overweight or obesity and existing cardiovascular disease, semaglutide 2.4 mg reduced the risk of major adverse cardiovascular events by 20% compared with placebo. On the strength of this, Wegovy is licensed in the UK for cardiovascular risk reduction in eligible patients — currently the only weight-loss injection with that licence. If you have established heart disease, this can be a deciding factor, and it's something your prescriber will weigh.
Side-by-side comparison
| Mounjaro | Wegovy | |
|---|---|---|
| Active ingredient | Tirzepatide | Semaglutide |
| How it works | GLP-1 + GIP receptor agonist (dual action) | GLP-1 receptor agonist |
| How it's taken | Once-weekly injection (KwikPen) | Once-weekly injection (FlexTouch pen) |
| Dose ladder | 2.5 / 5 / 7.5 / 10 / 12.5 / 15 mg | 0.25 / 0.5 / 1 / 1.7 / 2.4 mg |
| Average trial weight loss | Up to ~21% over 72 weeks (SURMOUNT-1) | ~15% over 68 weeks (STEP 1) |
| Cardiovascular licence | No | Yes — risk reduction in eligible patients (SELECT) |
Side effects: broadly similar
Because both medicines slow digestion and act on appetite, their side-effect profiles overlap heavily: nausea, constipation or diarrhoea, bloating and tiredness are the most common, particularly in the first weeks and after each dose increase. Both use gradual dose escalation specifically to keep side effects manageable. Individual tolerance genuinely differs — some people who struggle on one do noticeably better on the other.
How to choose — and who decides
Both medicines are prescription-only, so the decision is made with a prescriber after a clinical assessment of your BMI, medical history, medications and goals. Broadly:
- Maximum average weight loss in trials favours Mounjaro.
- Established cardiovascular disease may favour Wegovy, given its licensed heart benefit.
- Previous experience matters — if you've taken one before, your response and tolerance shape the recommendation.
- Practicalities like availability can also play a part.
If you start one and it doesn't suit you, switching is possible — switchers start at the lowest dose of the new medicine so the body can adjust safely. Read our guide to switching between Wegovy and Mounjaro for how that works.
The bottom line
There's no universally "better" injection — there's the one that's better for you. Mounjaro leads on average trial weight loss; Wegovy holds the cardiovascular licence and a longer real-world track record. A proper clinical consultation, not marketing, should settle it — and whichever you take, results depend on pairing the medicine with lasting changes to eating, movement and sleep.
Frequently asked questions
Is Mounjaro stronger than Wegovy?
In head-to-head and cross-trial comparisons, tirzepatide (Mounjaro) has produced greater average weight loss than semaglutide (Wegovy). Individual responses vary, and "strongest average" isn't the same as "best for you".
Can I switch from Wegovy to Mounjaro, or the other way?
Yes, with prescriber approval. Switchers restart at the lowest dose of the new medicine and titrate up, because the two active ingredients aren't interchangeable milligram for milligram.
Do Mounjaro and Wegovy have the same side effects?
Very similar: nausea, constipation, diarrhoea and fatigue are most common with both, especially after dose increases. Individual tolerance differs between the two medicines.
Which is better for people with heart disease?
Wegovy is currently the only one licensed in the UK for reducing cardiovascular risk in eligible patients, based on the SELECT trial. Your prescriber will weigh this against your full history.
References
- Wilding JPH, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). N Engl J Med. 2021;384(11):989–1002.
- Jastreboff AM, et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). N Engl J Med. 2022;387(3):205–216.
- Aronne LJ, et al. Tirzepatide as compared with semaglutide for the treatment of obesity (SURMOUNT-5). N Engl J Med. 2025.
- Lincoff AM, et al. Semaglutide and cardiovascular outcomes in obesity without diabetes (SELECT). N Engl J Med. 2023;389(24):2221–2232.
- Mounjaro KwikPen — Summary of Product Characteristics. medicines.org.uk.
- Wegovy — Summary of Product Characteristics. medicines.org.uk.