Decision guide · Evidence-led, sources cited
Mounjaro vs Wegovy
They are the two licensed weight-loss injections most UK adults are choosing between. Both are weekly injections; both work on the gut hormone system; both are backed by large trials. The honest summary: in the evidence we have, Mounjaro has produced greater average weight loss, but the right choice depends on your situation, your prescriber's assessment, and what you can access. Here is what's published.
| Mounjaro | Wegovy | |
|---|---|---|
| Active ingredient | Tirzepatide | Semaglutide |
| How it works | Dual GIP + GLP-1 receptor agonist | GLP-1 receptor agonist |
| Injection | Weekly, subcutaneous | Weekly, subcutaneous |
| Published trial weight loss | ~20.9% mean at 72 weeks, highest dose (SURMOUNT-1, NEJM 2022) | ~14.9% mean at 68 weeks (STEP 1, NEJM 2021) |
| Starting dose (per SmPC) | 2.5mg weekly for 4 weeks, then review | 0.25mg weekly for 4 weeks, then step up |
| Common side effects (per SmPC) | Nausea, diarrhoea, constipation, reduced appetite | Nausea, diarrhoea, constipation, reduced appetite |
What the evidence actually says
In their respective pivotal trials, tirzepatide (Mounjaro) showed a higher mean weight loss than semaglutide (Wegovy) — though these were separate trials, run differently, so direct comparison has limits. A head-to-head trial, SURMOUNT-5, subsequently reported greater average weight loss on tirzepatide than semaglutide. The evidence points toward Mounjaro for magnitude of weight loss; uncertainty remains at the individual level, where response varies.
How to choose
If access and cost are equal, the trial evidence leans toward Mounjaro for weight loss. But Wegovy has a longer real-world track record and may be easier to access or better tolerated for some people. Tolerability, your medical history, and supply all matter — and those are your prescriber's call, not a number on a page.
Switching between them
Switching from one to the other is common, but it is a clinical decision about timing and starting dose — not something to do on your own. Our switching guide covers what's published and what to ask.