Mounjaro®
Starting from £179.99/mo
Start journey Learn moreHow the side-effect profiles compare in practice, and why licensing matters for weight-loss patients
Step 1, Understanding the mechanisms that drive the side effects
Semaglutide (the active ingredient in Ozempic) and tirzepatide (Mounjaro) both slow gastric emptying and reduce appetite through the gut-hormone system. Slower gastric emptying is the single biggest driver of nausea, vomiting and the feeling of fullness that characterises both medicines. The NHS tirzepatide page and its semaglutide equivalent both list nausea, diarrhoea, constipation, vomiting, indigestion and fatigue as the most commonly reported effects.
Tirzepatide adds GIP receptor activation on top of GLP-1. Whether that second pathway worsens or moderates GI effects compared with GLP-1 alone is still an active question in the literature. What the trial data does show is that the pattern of side effects looks similar, but the absolute rates differ slightly, and those numbers come from different trial populations at different doses, so direct comparison needs care. If you want to understand how those differences play out in practice, our detailed look at Ozempic vs Mounjaro side effects covers that nuance in detail.
One practical point worth stating early: most GI symptoms are dose-dependent. They tend to peak in the days after starting or increasing a dose, then settle as the body adjusts. That is why both medicines use a slow titration schedule that begins well below the eventual maintenance dose.
Step 2 (Reading the comparison table (trial data, cited)
The figures below come from the pivotal phase-3 trials: SURMOUNT-1 for tirzepatide and the STEP programme for semaglutide 2.4 mg (Wegovy, not Ozempic) Ozempic's weight-relevant dosing was studied separately). Because Ozempic's licensed UK dose for diabetes is up to 1 mg, while Wegovy reaches 2.4 mg, direct side-effect comparisons against tirzepatide at 10 or 15 mg involve populations using different doses. The table reflects that honestly.
| Side effect | Tirzepatide 15 mg (SURMOUNT-1) | Semaglutide 2.4 mg (STEP 1) |
|---|---|---|
| Nausea | ~33% | ~44% |
| Diarrhoea | ~23% | ~30% |
| Vomiting | ~13% | ~24% |
| Constipation | ~17% | ~24% |
| Discontinuation due to GI events | ~4–5% | ~4–5% |
Sources: SURMOUNT-1, NEJM 2022; STEP 1, NEJM 2021. Numbers are approximate and reflect the highest maintenance doses studied; rates at lower doses are generally lower. Discontinuation rates due to GI events were broadly similar, which suggests tolerability (not just frequency) is comparable at therapeutic doses.
If you are weighing the broader picture of risks alongside side effects, our page on Ozempic and Mounjaro risks sets out what prescribers monitor for during treatment.
Step 3, Serious but infrequent effects both medicines share
In January 2026 the MHRA issued a Drug Safety Update specifically highlighting acute pancreatitis as a known but uncommon risk with GLP-1 receptor agonist medicines. The warning applies to both semaglutide and tirzepatide. The symptom to act on is severe, persistent stomach pain that radiates towards the back (with or without vomiting) especially if it does not pass quickly. That warrants urgent medical attention, not a wait-and-see approach.
Other effects flagged across both classes include gallbladder problems, dehydration from severe GI illness (which can place strain on the kidneys), and injection-site reactions. Anyone experiencing signs of a serious allergic reaction (swelling of the face or throat, difficulty breathing) should seek emergency care. Suspected side effects from either medicine can be reported at the MHRA's Yellow Card scheme, which the regulator actively monitors for emerging signals.
The Wegovy vs Mounjaro side effects comparison goes into these shared serious effects in more depth, because Wegovy and Mounjaro are the two licensed weight-loss medicines, the clinically relevant question for most people reading this page.
What is licensed for weight loss in the UK, and where to go from here
If you have landed on this page because you are considering treatment, the honest answer is that Ozempic is not the right starting point. It holds a UK licence for type 2 diabetes only; prescribing it for weight loss would be off-label. The medicines your prescriber can assess you for are Mounjaro (tirzepatide) and Wegovy (semaglutide in its weight-management formulation). Both are approved by the MHRA for adults with a BMI of 30 or above, or 27 or above with a weight-related condition, see NICE guidance for the full criteria at TA1026 (tirzepatide).
We understand the instinct to cross-compare every available medicine before committing to anything, that is a sensible approach to a decision that affects your health for months or years. Which one suits you depends on factors a table cannot settle: your other medicines, your medical history, how you feel about injections versus tablets, and how your body actually responds to the starter dose. Our Wegovy vs Mounjaro overview and the detail on cost differences between the two can help frame that conversation. Ultimately, though, which is right for you is a clinical decision our prescribers make with you, not one a webpage can make alone.
If you want to start that conversation, speak to our prescribers through a free consultation. A GPhC-registered Independent Prescriber reads your answers the same day (not software, a real clinician) and advises accordingly.
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