Mounjaro®
Starting from £179.99/mo
Start journey Learn moreHow the evidence separates two medicines that look alike on the surface
What the trial data actually show about their overlapping side effects
The shared ground between these two medicines is substantial. Both the STEP 1 trial for semaglutide 2.4mg (Wegovy) and the SURMOUNT-1 trial for tirzepatide (Mounjaro), published in the New England Journal of Medicine, recorded nausea as the most commonly reported side effect, followed by diarrhoea, vomiting and constipation. That pattern reflects what both medicines do: slow the rate at which food leaves the stomach, which reduces appetite but also unsettles the gut, particularly in the early weeks and after each dose increase.
The NHS medicines page for tirzepatide and its semaglutide equivalent both confirm this GI-led picture and note that symptoms are typically most noticeable at the start of treatment or when the dose steps up, often settling within days. Fatigue, headache and injection-site reactions also appear in both medicines' data. So the broad shape of what you might experience is genuinely comparable.
Where a misconception sometimes takes hold is the idea that one medicine is categorically "gentler" or "harsher" than the other. The trial populations, study lengths and dose schedules all differed, which makes direct frequency comparisons unreliable. If you're trying to work out how Wegovy and Mounjaro side effects compare in practical terms, the detail matters far more than the category.
Where the mechanisms create genuinely different risks
Mounjaro activates both GIP and GLP-1 receptors; Wegovy acts on GLP-1 receptors alone. That dual action is clinically relevant to how some side effects present. The NHS England guidance on weight-management injections specifically notes that tirzepatide's interaction with oral contraceptives warrants additional precautions: women using the pill should add a non-oral method of contraception for the first four weeks of tirzepatide treatment and for four weeks after each dose increase, because gastric slowing may reduce pill absorption. The same guidance does not flag an equivalent concern for semaglutide.
On pancreatitis, both medicines carry a known but infrequent risk. In January 2026 the MHRA issued a Drug Safety Update covering GLP-1 medicines as a class, highlighting acute pancreatitis as a serious possibility and advising anyone experiencing severe, persistent stomach pain that may radiate to the back to seek urgent medical attention. That warning applies to both Wegovy and Mounjaro equally. Gallbladder events, dehydration from severe GI illness and, in patients with diabetes, hypoglycaemia are also shared class considerations.
Neither medicine is recommended during pregnancy, breastfeeding or when actively trying to conceive, and neither is licensed for use in people under 18.
A side-by-side look at the key differences
| Feature | Mounjaro (tirzepatide) | Wegovy injection (semaglutide 2.4mg) |
|---|---|---|
| Mechanism | Dual GIP + GLP-1 receptor agonist | GLP-1 receptor agonist |
| Most common side effects | Nausea, diarrhoea, vomiting, constipation | Nausea, diarrhoea, vomiting, constipation |
| Oral contraceptive interaction | Add non-oral contraception for first 4 weeks of treatment and 4 weeks after each dose increase (NHS England guidance) | No equivalent absorption concern flagged |
| Pancreatitis risk | Infrequent but serious; seek urgent help for severe radiating stomach pain (MHRA DSU, Jan 2026) | Same class risk; same urgent-help advice |
| Average weight loss in trials | ~20–21% at 15mg over 72 weeks (SURMOUNT-1) | ~15% at 2.4mg over 68 weeks (STEP 1) |
| UK licence (weight management) | BMI ≥30, or ≥27 with a weight-related condition | BMI ≥30, or ≥27 with a weight-related condition |
For a fuller look at how the two medicines compare beyond side effects, the Wegovy vs Mounjaro overview on this site covers the broader picture, including the head-to-head SURMOUNT-5 trial evidence.
What this means when you're choosing between them
The honest answer is that no one can predict from the outside which medicine your system will tolerate better. People who want to understand whether Wegovy tends to produce fewer side effects than Mounjaro will find a detailed breakdown worth reading before making a decision. Trial averages describe populations, not individuals.
That is precisely why a prescriber's assessment matters. At nume, our clinical team reviews your health history, current medications and weight-related conditions before recommending a treatment. The side-effect comparison for Mounjaro and Wegovy goes into further depth on individual symptoms, and the cost comparison page covers what you'd pay for each through a private regulated pharmacy. If you have specific concerns about tolerability, including whether Wegovy actually carries more side effects than Mounjaro, the FAQs address the questions our prescribers hear most often.
Which of these medicines suits you is a clinical decision our prescribers make with you, never a guess, never a generic recommendation. You can start your free consultation and have your answers reviewed the same working day.
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