Mounjaro®
Starting from £179.99/mo
Start journey Learn moreHow the side-effect profiles of Wegovy and Mounjaro actually differ, and where they overlap
Step 1: understand that both medicines cause GI side effects, the question is degree and pattern
Wegovy (semaglutide) is a GLP-1 receptor agonist; Mounjaro (tirzepatide) activates both GLP-1 and GIP receptors. Because the GLP-1 pathway is what slows gastric emptying and reduces appetite, both medicines produce a GLP-1-driven side-effect signature: nausea, vomiting, diarrhoea, constipation, indigestion and burping. These tend to be most noticeable in the early weeks of treatment or after a dose increase, and most people find they settle as the body adjusts. Neither medicine eliminates this GI burden, though the degree varies between individuals. The shared side-effect picture is more striking than the differences, which is worth knowing before you read too much into head-to-head percentages. If you want to get straight to the question of whether Wegovy tends to cause more side effects than Mounjaro, that comparison is covered in detail on its own page. The NHS medicines pages for tirzepatide and semaglutide both list very similar GI effects as the most common category.
One practical note: Mounjaro's dual-receptor action means it does not simply replicate Wegovy's pharmacology. Whether activating the GIP pathway meaningfully changes tolerability in practice is still an active area of research. The short answer for most patients is: the GI experience is comparable, but individual variation is large enough that a person who struggles on one may tolerate the other better.
Step 2: read the trial data carefully, the studies were not designed to compare side-effect rates directly
The SURMOUNT-1 trial (tirzepatide, published in the New England Journal of Medicine) and the STEP 1 trial (semaglutide 2.4mg) both reported GI adverse events in roughly 40–80% of participants, with most rated mild to moderate. Crucially, these trials used different populations, different follow-up periods and different dose-escalation schedules, so comparing percentages between them is not a fair apples-to-apples exercise. The SURMOUNT-5 head-to-head trial compared tirzepatide and semaglutide 2.4mg directly on weight outcomes, but it was not powered to draw definitive conclusions about tolerability differences. For a deeper look at whether semaglutide produces fewer side effects than tirzepatide overall, that evidence is genuinely limited.
What the data does show consistently is that injection-site reactions occur with both medicines. Dizziness, fatigue and headache are also reported across both programmes. Serious events (including acute pancreatitis, gallbladder problems and allergic reactions) are rare but flagged for both; the MHRA issued a Drug Safety Update in January 2026 highlighting acute pancreatitis as an infrequent but potentially serious risk with GLP-1 medicines. Severe, persistent abdominal pain that spreads to the back warrants urgent medical attention regardless of which medicine you are taking.
Step 3: use this comparison table as a starting point, not a verdict
| Feature | Wegovy (semaglutide 2.4mg) | Mounjaro (tirzepatide up to 15mg) |
|---|---|---|
| Mechanism relevant to side effects | GLP-1 receptor agonist | Dual GLP-1 and GIP receptor agonist |
| Most common side effects | Nausea, vomiting, diarrhoea, constipation, indigestion | Nausea, vomiting, diarrhoea, constipation, indigestion |
| GI side-effect timing | Typically peaks early or after dose increase; often settles over weeks | Typically peaks early or after dose increase; often settles over weeks |
| Injection-site reactions | Reported; rotate sites to reduce | Reported; rotate sites to reduce |
| Serious events (rare) | Pancreatitis, gallbladder problems, allergic reaction [MHRA DSU Jan 2026] | Pancreatitis, gallbladder problems, allergic reaction [MHRA DSU Jan 2026] |
| Contraception interaction | No equivalent evidence of reduced pill effectiveness | Add non-oral contraception for first 4 weeks and after each dose increase [NHS England] |
The contraception difference is worth highlighting. NHS England advises that women taking oral contraceptives should use an additional non-oral method (such as condoms) for the first four weeks on Mounjaro and for four weeks after each dose increase, because absorption of the pill may be reduced. No equivalent guidance applies to Wegovy. For anyone weighing the side-effect picture between these two medicines, that practical consideration can matter as much as any GI percentage.
Step 4: which medicine is right for you is a clinical decision, not a label comparison
People searching whether Mounjaro or Wegovy has fewer side effects often find an inconclusive answer because the honest one is: it depends on the person. Titration speed, starting dose, baseline health, other medicines and individual gut sensitivity all shape tolerability more than the brand name does. Our prescribers at nume look at the full picture before recommending either. They review your health history, current medicines (including contraception and HRT) and your weight-loss goals before any decision is made. That review happens the same working day; your consultation lands on a named prescriber's desk, not in a queue for an algorithm.
If you are already on one of these medicines and finding the side effects difficult, do not adjust your dose without speaking to your prescribing team first. The cost difference between Wegovy and Mounjaro is another factor many people weigh up at this stage, worth reading alongside the clinical picture. You can also explore the range of treatment options we support, or go straight to starting your free consultation and let our prescribers work through it with you.
Looking to start your weight loss journey?
Take a quick eligibility quiz to explore your options and see how we can support you.