Semaglutide vs Mounjaro: How Do These Two Medicines Compare?

Do semaglutide and Mounjaro work in the same way?

Not quite. Semaglutide is a GLP-1 receptor agonist: it mimics a gut hormone that slows digestion, reduces appetite and helps regulate blood sugar. Mounjaro (tirzepatide) does all of that, but also activates the GIP receptor, a second gut-hormone pathway involved in appetite and metabolism. This dual action is what makes tirzepatide the only medicine of its kind licensed for weight management in the UK, according to NHS guidance on tirzepatide.

In practice, both medicines slow gastric emptying and reduce how much you feel like eating. The difference is in degree. Clinical trials suggest that activating both pathways produces larger average weight reductions, though individual responses vary considerably. Neither medicine is a passive treatment: both are designed to be used alongside a reduced-calorie diet and increased physical activity.

It's also worth knowing what semaglutide is not. Ozempic is semaglutide too, but it's licensed for type 2 diabetes, not weight loss. For weight management, the licensed UK injection is Wegovy. There's now also an oral Wegovy tablet, approved by the MHRA in June 2026, which is a separate option worth understanding, and if you want to compare the oral semaglutide route with Mounjaro specifically, our Rybelsus vs Mounjaro page sets out how those two options differ. The Mounjaro vs Ozempic comparison explains the diabetes/weight-loss distinction in more detail if you've seen Ozempic mentioned elsewhere.

What do the trial results show for each medicine?

The most useful comparison comes from SURMOUNT-5, an open-label head-to-head trial of 751 adults with obesity and no diabetes, run over 72 weeks. Tirzepatide produced a greater average body-weight reduction than semaglutide 2.4mg. NICE's appraisal notes in TA1026 that indirect comparisons also favour tirzepatide.

Looking at each medicine's own pivotal trial: SURMOUNT-1 (2,539 participants, 72 weeks) showed average weight reduction of around 20–21% at the 15mg tirzepatide dose. STEP 1 (semaglutide 2.4mg, 68 weeks) showed around 15% average reduction. The newer semaglutide 7.2mg dose (approved by the MHRA in early 2026) narrows the gap, with around 20.7% average loss reported in trials, and our tirzepatide vs semaglutide comparison walks through what these converging numbers mean when choosing between the two. So the picture is not static.

Numbers from SURMOUNT-1 (NEJM), STEP 1 and MHRA approval communications. Trial averages are not individual predictions.

Are the side effects and eligibility criteria the same for both?

The side-effect profiles overlap substantially. Nausea, loose stools, constipation, indigestion and fatigue are the most common with both medicines, typically peaking around dose increases and settling over days to a couple of weeks. Injection-site reactions can occur with either. The NHS semaglutide medicine page and the equivalent tirzepatide page both carry full side-effect lists worth reading before you start.

There is one practical difference worth flagging for people on oral contraceptives. NHS guidance notes that tirzepatide can reduce pill absorption during the first four weeks of treatment and for four weeks after each dose increase; an additional non-oral method is advisable during those windows. The same concern doesn't have equivalent evidence for semaglutide. If you take HRT, the same absorption question applies to tirzepatide, and transdermal options are worth discussing with your prescriber.

Eligibility is broadly similar for private prescribing: adults with a BMI of 30 or above, or 27 or above alongside at least one weight-related condition such as high blood pressure, high cholesterol or type 2 diabetes. Lower thresholds apply for some ethnic backgrounds under UK guidance. Neither medicine is recommended during pregnancy, breastfeeding or when trying to conceive, and neither is licensed for under-18s. A full medical history matters, which is why clinical assessment comes before any prescription. For a closer look at whether Mounjaro or Wegovy comes out ahead for your situation, the detailed breakdown of Mounjaro vs semaglutide for weight loss covers the nuances our prescribers weigh up.

Which one should you choose, and how does the consultation help?

There is no universally correct answer, and anyone who tells you otherwise is oversimplifying. Average trial data is genuinely useful context; it is not a prediction for any specific person. Your starting weight, existing conditions, any medicines you take, how you respond to the starting dose, and practical factors like refrigeration or travel all feed into the decision.

The prescribers at our clinical team review every case individually. For people wondering specifically whether to switch between the two, the guide to switching from Mounjaro to semaglutide is a good starting point, but that decision always involves a clinical review, not a self-directed change. If cost is part of your thinking, this page on Mounjaro vs Wegovy pricing gives an honest comparison of what private treatment typically costs for each. For a broader look at the treatments available through nume, the weight-loss treatments page is the place to start your free consultation.