Mounjaro vs Wegovy Safety: What the Clinical Evidence Actually Shows

What the SURMOUNT and STEP trials tell us about tolerability

The clearest safety comparison comes from the trials that earned each medicine its UK licence. In SURMOUNT-1, published in the New England Journal of Medicine, tirzepatide's most commonly reported side effects were gastrointestinal: nausea, diarrhoea, vomiting and constipation. Rates were broadly similar across the 5mg, 10mg and 15mg doses, and most events were mild to moderate. In the STEP 1 trial for semaglutide 2.4mg, the GI profile was almost identical in character (nausea, diarrhoea, vomiting) at broadly comparable rates.

The head-to-head SURMOUNT-5 trial (72 weeks, published in the New England Journal of Medicine in 2025) gave us the most direct comparison available. Tirzepatide produced greater average weight reduction than semaglutide 2.4mg, but the trial was not powered to show a difference in side-effect incidence between the two medicines. Both arms reported similar GI tolerability. That finding is consistent with what the SmPCs for each product describe.

The practical upshot: neither medicine is obviously easier on the stomach than the other at the population level. Individual responses vary, which is one reason the prescriber's assessment of your starting point matters, and our guide to which of the two tends to cause fewer side effects can help frame that conversation.

Where the safety profiles genuinely differ

Mechanism is the main dividing line. Mounjaro (tirzepatide) activates both GIP and GLP-1 receptors; Wegovy (semaglutide) activates GLP-1 alone. That dual action is why tirzepatide produces a larger average weight reduction in trials, but it also introduces one safety distinction worth knowing: the absorption of oral medicines, including the combined oral contraceptive pill, may be reduced during the first four weeks of tirzepatide treatment and for four weeks after each dose increase. NHS England's guidance on weight-management injections advises adding a non-oral contraceptive method (such as condoms) during those windows. No equivalent signal has emerged for semaglutide. For the same reason, NHS England suggests discussing a switch to transdermal HRT (patches or gels) with your doctor if you are on tirzepatide.

Both medicines carry a Black Triangle (▼) designation in the UK, meaning additional MHRA monitoring applies. Both are contraindicated in pregnancy, breastfeeding, and for people trying to conceive. Neither is licensed for under-18s. People with a personal or family history of medullary thyroid carcinoma or MEN2 should not use either medicine; pancreatitis history requires careful prescriber discussion.

Gallbladder problems (including gallstones) appear in the safety information for both medicines, as they do for other GLP-1 class treatments. If you would like a fuller picture of how the two medicines compare across tolerability, our detailed look at Mounjaro and Wegovy side effects covers both medicines in one place, alongside results, mechanism and eligibility. If you'd like to read about how the two medicines compare beyond safety, our Wegovy vs Mounjaro overview covers results, mechanism and eligibility in one place.

The MHRA pancreatitis warning and what to watch for on either medicine

A question our prescribers hear most weeks is whether one medicine carries a higher pancreatitis risk than the other. The honest answer is that the MHRA's January 2026 Drug Safety Update on GLP-1 medicines (which flagged acute pancreatitis as an infrequent but serious class-level risk) applies to both tirzepatide and semaglutide without distinguishing between them. The symptom to act on immediately is severe, persistent abdominal pain that radiates to the back, with or without vomiting. That symptom warrants urgent medical attention regardless of which medicine you are taking.

Both medicines slow gastric emptying, which can occasionally amplify GI symptoms and, in people who develop significant nausea or vomiting, raise a secondary concern about dehydration. Staying well hydrated and eating smaller, lower-fat meals during the initial weeks reduces most of that risk in practice. Suspected side effects from either medicine can be reported directly at the MHRA's Yellow Card scheme, patients can do this themselves, not only clinicians.

For a more granular breakdown of which side effects are more or less frequent at each dose level, our side-by-side look at how Wegovy and Mounjaro side effects compare across both medicines works through the SmPC data systematically. If you are already on one medicine and are weighing a change, our page on switching safely between Mounjaro and Wegovy covers the clinical considerations involved.

Side-by-side safety summary

The table captures the headline comparison. It does not replace a prescriber's assessment of your own health history, current medicines and individual risk factors. Which medicine is the right fit for you is a clinical decision our prescribers make with you, not one that can be settled by a comparison page alone. If you are ready to take that step, you can start your free consultation today.