Mounjaro and Wegovy side effects: what's different, what's the same

The side effects Mounjaro and Wegovy share

The most common reactions to both medicines sit in the gut. Nausea is the one patients mention most often, it tends to be sharpest in the first week or two after starting, or after each dose step up, then settles for most people. Vomiting, loose stools, constipation, burping, and an uncomfortable full feeling after small meals are also reported with both. Fatigue and headaches appear in the trial data for each medicine, as does occasional dizziness.

That shared profile makes sense. Mounjaro activates both GIP and GLP-1 receptors; Wegovy targets GLP-1 alone. Both pathways slow gastric emptying, which is largely what drives the nausea and bloating. The question of which causes fewer side effects is genuinely difficult to answer in the abstract, the answer varies by person and by dose. The NHS medicines pages for both tirzepatide and semaglutide list these GI effects under common reactions alongside injection-site redness or itching, which is specific to both injectable formats.

One important safety note applies to both: the MHRA highlighted acute pancreatitis as a known, if infrequent, serious risk with GLP-1 medicines. Severe, persistent stomach pain that seems to reach through to the back (with or without vomiting) means stopping the injection and seeking urgent medical attention. Don't wait to see if it passes.

Where the two medicines differ in their side-effect picture

Mounjaro's dual mechanism appears to produce stronger appetite suppression on average, which may explain why some people find the GI effects feel more pronounced, particularly early on. The titration schedule starts at 2.5 mg (a dose whose main job is helping your system adjust rather than driving weight loss) precisely because the higher doses can cause significant nausea if introduced too quickly.

For Wegovy, the escalation from 0.25 mg up to 2.4 mg (and now optionally to 7.2 mg, following the MHRA's approval of the higher-dose pen in April 2026) follows a similar philosophy of gradual tolerance-building. Because semaglutide acts on a single receptor pathway, some clinical teams find it slightly easier to predict tolerability, though this is not a hard rule, individual responses vary considerably.

There is also a practical difference for women taking oral contraceptives. NHS guidance advises that tirzepatide may reduce pill absorption during the first four weeks of treatment and for four weeks after each dose increase; an additional non-oral method is recommended during those windows. The same concern has not been shown with semaglutide. Likewise, NHS England suggests considering transdermal HRT (patches or gels rather than tablets) while on Mounjaro, for a similar absorption reason. These are not reasons to avoid either medicine; they're decisions to make with a prescriber who knows your full picture. If this is your main concern, you can read a detailed side-by-side look at how Wegovy and Mounjaro compare, covering both the shared reactions and the differences that may matter most to you.

Side-effect comparison at a glance

Frequency categories (very common: affects more than 1 in 10; common: 1 in 10 to 1 in 100) are drawn from the respective SmPCs. Both medicines carry a Black Triangle (▼) status with the MHRA, meaning they are subject to additional monitoring.

What this means when you're deciding between them

Side effects alone rarely settle the choice. The SURMOUNT-5 head-to-head trial, published in the New England Journal of Medicine in 2025, found tirzepatide produced greater average weight loss than semaglutide 2.4 mg at 72 weeks, though Wegovy's 7.2 mg dose narrows that gap. How your body responds to either medicine is something no comparison table can predict. Factors like other medicines you take, your contraception, any planned surgery (you'd need to tell the anaesthetist), and your medical history all feed into the clinical picture.

Keeping your pen in the fridge door is a small but useful routine that reminds you it needs refrigerating, both medicines require cold storage, and the exact room-temperature window for each is detailed in the patient information leaflet that comes with your pen. Never rely on a memory of a number read online for something as important as storage.

A prescriber who has reviewed your full history is the right person to weigh this up with you, not a side-effect table. At nume, that review happens the same day, carried out by a named GPhC-registered prescriber. See the evidence on Wegovy's side-effect profile, read about what the trials actually show on tolerability, or browse our treatment overview, then, when you're ready, start your free consultation and let our clinical team help you decide. You can also explore a full breakdown of how Wegovy and Mounjaro compare on side effects, including both the reactions they share and those that set them apart.