Fewer side effects with Mounjaro or Wegovy — what the evidence actually shows

You're weighing up two options and the nausea question is front of mind

Say you've done some reading, you know both are once-weekly injections, and you've landed on the same concern almost everyone does: which is going to make me feel sick? It's a fair question. Gastrointestinal effects (nausea, loose stools, constipation, reflux, belching) are the dominant side-effect pattern for both tirzepatide (Mounjaro) and semaglutide (Wegovy), because both drugs slow gastric emptying as part of how they work.

The misconception worth letting go of gently is that one of them is essentially fine and the other is rough. Neither is. Both medicines cause GI symptoms in a meaningful proportion of users, particularly in the first weeks of a new dose. The symptoms are usually temporary, typically settling within a week or two as the body adjusts, and they're one of the main reasons both are started at a low tolerability dose before the prescriber steps things up.

What the trials do show is that the severity and frequency of individual effects differ slightly. In the SURMOUNT-1 trial of tirzepatide, nausea affected around 29–31% of participants depending on dose; in the STEP 1 trial of semaglutide 2.4mg, nausea was reported in roughly 44% of the active group. Those figures come from different study populations and designs, so a head-to-head read requires caution. The direct side-effect comparison between Wegovy and Mounjaro in the SURMOUNT-5 trial (the first genuine head-to-head) gives a cleaner picture, discussed below. You can also check the NHS's patient-level guidance on tirzepatide side effects for a plain-English breakdown.

Where the two medicines differ: mechanism, rate of GI effects, and one notable distinction

Wegovy targets a single receptor: GLP-1. Mounjaro hits two, GIP and GLP-1, which is part of why the weight-loss results differ, but the dual mechanism also changes the side-effect picture subtly. In SURMOUNT-5, which ran the two medicines head-to-head for 72 weeks in adults with obesity and no diabetes, rates of GI-related discontinuation were broadly similar between tirzepatide and semaglutide 2.4mg, suggesting neither medicine is dramatically better tolerated than the other overall, even if individual symptoms land differently. The SURMOUNT-5 results were published in the New England Journal of Medicine in 2025 and inform NICE's current thinking on the two treatments.

One distinction that comes up in clinical practice: the injection-site reactions. Both are subcutaneous injections, but some people report redness, itching or localised discomfort more with one than the other, this tends to be idiosyncratic rather than a reliable predictor. Rotating the injection site (abdomen, thigh, upper arm) helps with both. There is also the oral Wegovy tablet, approved by the MHRA in June 2026, which removes the injection entirely; GI side effects in the OASIS 4 trial affected around 74% of participants, though most were mild to moderate and transient. If you've read that Wegovy has fewer side effects than Mounjaro, that page looks carefully at what the evidence does and doesn't support. For a fuller look at how the two medicines compare on tolerability, that page goes deeper.

The MHRA's Yellow Card scheme lets anyone report a suspected side effect from either medicine, patients can do this directly, and it contributes to ongoing post-marketing safety data for both.

Side-effect comparison at a glance

These figures come from separate trials with different populations; direct numerical comparison should be treated as indicative only. Sources: NEJM SURMOUNT-1; NEJM STEP 1.

What actually determines your experience, and how a prescriber helps

Tolerability is personal. Someone who finds semaglutide intolerable may do fine on tirzepatide, and vice versa. The factors that shape this include the pace of dose titration (slower increases tend to reduce GI burden), what you eat alongside treatment, timing of the dose, and individual gut sensitivity. Both medicines have a titration schedule that exists precisely to let the body adapt; pushing through dose steps faster than the prescriber recommends reliably makes side effects worse.

There are also factors beyond GI symptoms worth flagging to a prescriber: the MHRA issued guidance in January 2026 highlighting acute pancreatitis as an infrequent but serious risk for GLP-1 medicines, severe stomach pain that radiates to the back needs urgent attention. That applies to both Mounjaro and Wegovy. Pregnancy, breastfeeding, and trying to conceive are all contraindications for either medicine; women on oral contraceptives starting Mounjaro specifically are advised to add a barrier method for the first four weeks of treatment and after each dose increase, as tirzepatide may reduce pill absorption.

Which of these medicines produces fewer side effects for you is the kind of clinical question that our prescribers work through with each patient. If you're weighing up the two, our detailed tolerability guide covers the nuance, and the cost difference between Mounjaro and Wegovy may also be relevant to your decision. For an overview of both treatments, our weight-loss treatment page is a good place to start. Which medicine suits you is a clinical decision our prescribers make with you, speak to our prescribers through a free consultation.