Mounjaro®
Starting from £179.99/mo
Start journey Learn moreThe clinical evidence, the practical differences, and how to choose
Picture this: you've been told to try one of these, but you're not sure why
A prescriber has mentioned Mounjaro or Wegovy, or you've been on one and wondered about the other. Both are once-weekly injections. Both work by mimicking gut hormones that signal fullness and slow gastric emptying. The difference is the number of receptors involved. Wegovy contains semaglutide, a GLP-1 receptor agonist. Mounjaro contains tirzepatide, which activates both the GLP-1 and GIP receptors, making it the only dual-agonist weight-loss medicine currently licensed in the UK.
That dual action appears to matter in terms of results. In SURMOUNT-1, published in the New England Journal of Medicine, adults taking the highest tirzepatide dose lost around 20–21% of body weight on average over 72 weeks. STEP 1, the equivalent semaglutide 2.4mg trial, found around 15% average loss over 68 weeks. The gap narrowed somewhat with the approval of a higher 7.2mg semaglutide dose in 2026, with that formulation producing around 20.7% loss in trials, approaching, but not quite reaching, the tirzepatide figures. If you are weighing up the two options and want a detailed breakdown, our guide to what's best, Mounjaro or Wegovy, covers the key considerations before you assume the answer is obvious.
Neither result is typical of every person. These are population averages from controlled trials, and individual responses vary considerably depending on starting weight, adherence, diet, and activity levels.
Where Mounjaro and Wegovy actually differ day to day
The practical comparison between Wegovy and Mounjaro goes beyond the headline numbers. Here is a factual summary drawn from NICE's appraisal of tirzepatide (TA1026) and the published SmPCs.
| Factor | Mounjaro (tirzepatide) | Wegovy (semaglutide) |
|---|---|---|
| Mechanism | Dual GIP + GLP-1 agonist | GLP-1 agonist |
| Average weight loss (highest licensed dose) | ~20–21% (SURMOUNT-1, 72 weeks) | ~15% at 2.4mg (STEP 1, 68 weeks); ~20.7% at 7.2mg (2026 trials) |
| UK strengths | 2.5mg → up to 15mg | 0.25mg → up to 7.2mg |
| Head-to-head evidence | SURMOUNT-5 (NEJM, 2025): tirzepatide produced greater average weight reduction than semaglutide 2.4mg over 72 weeks in adults with obesity and no diabetes | |
| Oral contraceptive interaction | Add a non-oral method for the first 4 weeks of treatment and 4 weeks after each dose increase | No equivalent interaction currently documented |
| NICE recommendation | TA1026: BMI ≥35 + ≥1 weight-related comorbidity (private licence: BMI ≥30, or ≥27 with a condition) | TA875: BMI ≥35 + ≥1 comorbidity, within a specialist service, up to 2 years |
One practical point that catches people out: if you are taking an oral contraceptive pill and starting Mounjaro, you need to add a barrier method during the early weeks of treatment and after each dose increase because tirzepatide can reduce how much of the pill your body absorbs. Semaglutide does not carry the same documented risk. Your prescriber will ask about this. If you are also on HRT, NHS England advises discussing transdermal HRT with your doctor when taking tirzepatide.
Side effects: similar profiles, same questions from patients
Both medicines share a GI-led side effect profile. Nausea, loose stools, constipation, reflux and occasional vomiting are the most common. These tend to be most noticeable in the first few weeks after starting or moving up a dose, and they settle for most people over time. Headache, fatigue and injection-site reactions are also reported. The NHS patient page for tirzepatide and the equivalent page for semaglutide set out the full lists clearly.
The serious event to know about is pancreatitis, severe, persistent stomach pain that may spread to the back, with or without vomiting, requires urgent medical help. The MHRA highlighted this risk across GLP-1 medicines in a Drug Safety Update in January 2026. It is infrequent but can be serious. If you want a fuller comparison of safety profiles, our page on which is safer goes into more detail.
Neither medicine is recommended during pregnancy, while breastfeeding, or when trying to conceive. Treatment is not licensed for under-18s. History of certain thyroid conditions or pancreatitis requires a careful prescriber conversation before either is considered.
So which one should you take?
The evidence from SURMOUNT-5 gives tirzepatide a lead over semaglutide 2.4mg in a direct comparison. That makes Mounjaro look like the obvious pick, and for many patients it is. But the right answer genuinely depends on things a headline cannot account for: which medicines you already take, your thyroid history, whether you need a product available on the NHS or privately, how you'd manage the oral contraceptive interaction, and whether you've tried one already without the results you hoped for. If you're curious about how Mounjaro compares to other semaglutide products, our Mounjaro vs Ozempic page is worth reading too, though Ozempic is licensed for type 2 diabetes rather than weight loss.
Switching is also a real scenario. If you've been on Wegovy and want to know whether moving to Mounjaro is straightforward, the honest answer is that it requires clinical review rather than a simple swap. Read more about switching between the two before making any changes. Similarly, the price difference between them is a fair question, and the reasons Wegovy can cost less are worth understanding before assuming cheaper means better value. If you're placing an order around the end of the month, our Monday-to-Friday same-day dispatch means even a payday order can reach you the next working day.
Which medicine suits you best is a clinical decision our prescribers make with you. Speak to our prescribers through a free consultation and get a same-day assessment from a qualified professional, not an automated form.
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