Ozempic vs Mounjaro Side Effects: What Clinical Evidence Shows

What the NICE appraisal and trial data tell us about GI tolerability

NICE's appraisal of tirzepatide (TA1026, published December 2024) and its earlier appraisal of semaglutide for weight loss both drew on large phase-3 programmes. The SURMOUNT-1 trial (tirzepatide's flagship study, run in 2,539 adults with obesity) reported that nausea, diarrhoea, vomiting and constipation were the most common adverse events, occurring predominantly around dose increases and generally settling within one to two weeks. The STEP 1 trial for semaglutide 2.4mg (Wegovy) showed an almost identical rank order of GI complaints, with nausea again the most frequent.

Where numbers appear in indirect comparisons, they tell a broadly similar story: somewhere between 30% and 45% of participants in each programme reported nausea at some point, with fewer experiencing vomiting or diarrhoea persistent enough to discontinue. NICE's committee noted that direct head-to-head tolerability data between tirzepatide and semaglutide were limited at appraisal; the SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, focused on weight outcomes rather than a formal safety comparison. If you want to explore how Wegovy and Mounjaro compare on side effects in detail, that evidence is laid out alongside the trial data in one place. For a detailed look at how Mounjaro and Wegovy compare on side effects specifically, the are Wegovy side effects worse than Mounjaro page walks through that evidence.

One mechanistic point worth understanding: tirzepatide activates both GLP-1 and GIP receptors, whereas semaglutide acts on GLP-1 alone. Some researchers have suggested the GIP component may modulate nausea, potentially making tirzepatide's GI effects slightly more manageable at equivalent weight-loss doses, but this remains an area of ongoing research rather than a settled clinical fact. The NHS medicines page for tirzepatide lists the same GI-led side-effect profile you would find described for semaglutide.

The side effects that matter beyond nausea

Nausea dominates the conversation, but it is not the only consideration. Both medicines carry a prescriber-assessed risk of acute pancreatitis. In January 2026 the MHRA issued a Drug Safety Update for GLP-1 medicines highlighting this: severe, persistent stomach pain that radiates to the back, with or without vomiting, warrants urgent medical attention. That warning applies to both semaglutide and tirzepatide. You can report any suspected side effect through the MHRA Yellow Card scheme.

Gallbladder problems, including gallstones, appear in both programmes. Injection-site reactions are specific to the injectable forms of both medicines and generally mild. Headache, dizziness and fatigue appear in both datasets. Tirzepatide's broader side-effect risks (and how they differ from those of the semaglutide injection) are covered on the tirzepatide vs Ozempic side effects comparison page. If you want to read about the overlapping risks across both medicines in more depth, the Ozempic and Mounjaro risks page consolidates that picture.

One practical difference: Mounjaro starts at 2.5mg, a dose whose primary function is settling your system before any meaningful weight-loss effect begins. That graduated approach is reflected in the trial data, treatment discontinuation due to side effects tended to happen at lower rates with tirzepatide than historical semaglutide trials, though cross-trial comparisons should always be treated with caution.

Why Ozempic specifically is not the right reference point for weight-loss decisions

Ozempic delivers semaglutide at doses up to 1mg or 2mg, calibrated for blood-glucose control in people with type 2 diabetes. Wegovy delivers the same molecule at 2.4mg (or now up to 7.2mg under the newer licensed dose) specifically for weight management. The side-effect experience at those higher doses differs from the Ozempic experience, so comparing Ozempic side effects directly with Mounjaro side effects is not the most clinically useful frame if your goal is weight management.

The medicines licensed in the UK for weight loss are Wegovy (semaglutide) and Mounjaro (tirzepatide). Ozempic is not one of them, and exploring the licensed weight-loss treatment options gives a clearer picture of what is actually available. For a cost comparison alongside the clinical one, the tirzepatide vs semaglutide cost page sets out the private pricing context factually.

Side-effect comparison at a glance

The evidence reviewed by NICE does not establish that one medicine is meaningfully more tolerable than the other across all patients. Individual response varies, which is exactly why a prescriber reviews your history before recommending a starting point. If you're ready to have that conversation, speak to our prescribers through a free consultation. Our clinical team, including our clinical lead, reviews every consultation personally, your answers land on a prescriber's desk, not in an automated queue. Once approved, your pen arrives via DPD the next working day in plain, unbranded packaging, with tracking on your phone. That's the same-day review promise, end to end.