How tirzepatide and semaglutide differ — and why it matters for your treatment

What does the dual-receptor mechanism actually do differently?

Semaglutide (sold for weight management as Wegovy) mimics GLP-1, a hormone released after eating that signals fullness to the brain, slows the passage of food through the stomach and reduces appetite. Tirzepatide (sold as Mounjaro) does all of that, but also mimics GIP — a second gut hormone that works alongside GLP-1 on appetite, fat storage and insulin response. The combined activation of both receptors produces a stronger appetite-suppressing effect than GLP-1 alone, which is reflected in the trial data.

Both medicines are subcutaneous injections taken once a week, and both slow gastric emptying, which is why the side-effect profile is similar: nausea, loose stools and indigestion are common at the start and after each dose increase, typically settling within a week or two. The difference between Wegovy and Mounjaro in everyday use is therefore more about degree than kind, the same family of medicines, but tirzepatide's dual mechanism appears to produce larger average weight reductions at the top of the dosing range.

Crucially, tirzepatide is the only dual GIP and GLP-1 receptor agonist licensed for weight management in the UK, and if you're wondering how closely Mounjaro resembles Wegovy, the short answer is that they share the same weekly injection format and many of the same side effects, but differ meaningfully in how they work at receptor level, as confirmed by its product information on the Electronic Medicines Compendium (eMC). Semaglutide was first, and the clinical experience behind it is longer, both facts matter when a prescriber weighs up your individual picture.

What do the clinical trials show, head to head?

No two clinical programmes are perfectly comparable (different populations, different durations, different control arms) but the SURMOUNT and STEP trials give us the clearest picture available.

In SURMOUNT-1, tirzepatide at its highest dose produced an average body-weight reduction of around 20 to 21 percent over 72 weeks in adults with obesity and no diabetes, as published in the New England Journal of Medicine. STEP 1 showed semaglutide 2.4mg produced an average reduction of around 15 percent over 68 weeks in a similar population.

The most direct comparison comes from SURMOUNT-5 (2025), an open-label head-to-head trial in 751 adults with obesity and no diabetes: tirzepatide produced greater average weight reduction than semaglutide 2.4mg over 72 weeks. NICE's appraisal of tirzepatide (TA1026) notes that indirect comparisons consistently favour tirzepatide, and the committee considered SURMOUNT-5 the most relevant direct evidence.

The gap narrows with higher-dose semaglutide. The 7.2mg Wegovy pen, approved by the MHRA in April 2026, achieved around 20.7 percent average weight loss in its trial, approaching the tirzepatide 15mg figure. So the evidence picture is not static. For a fuller breakdown of the numbers side by side, the Wegovy vs Mounjaro comparison goes through each trial in context.

Are there differences in who can use them and how they're monitored?

The licensed eligibility criteria are very similar: both medicines are approved for adults with a BMI of 30 or above, or 27 to 29.9 with at least one weight-related condition such as high blood pressure, raised cholesterol or type 2 diabetes. Lower thresholds can apply for some ethnic backgrounds under UK guidance, and a prescriber assesses the whole picture rather than BMI alone.

One practical difference is the oral contraceptive interaction. If you take a combined pill, tirzepatide's slowing of gastric emptying may reduce how much of it is absorbed during the early weeks of treatment and after each dose increase; UK guidance recommends adding a non-oral method during those windows. Semaglutide does not carry the same evidence-based caution, though prescribers will review your full medication list regardless. Our prescribers discuss this at consultation, see our FAQs for more on what the review covers.

Both carry a Black Triangle (▼) designation, meaning the MHRA collects additional safety data on an ongoing basis. Side effects to watch for on either medicine include severe or persistent stomach pain that spreads to the back, seek urgent medical attention if that happens, and report any concerns via the MHRA Yellow Card scheme. Neither medicine is recommended during pregnancy, breastfeeding, or when trying to conceive, and neither is licensed for people under 18.

Which should you choose, and how does the decision get made?

The honest answer is that neither medicine is unconditionally better. Tirzepatide's dual mechanism produces larger average weight reductions across the trial evidence we have. Semaglutide has a longer track record, and at 7.2mg its results now sit much closer to tirzepatide's top-dose figures. For some people, a health condition, another medicine they take, or a previous experience with one of these treatments makes the choice straightforward. For others, it genuinely is a close call.

That call belongs with a prescriber, not a comparison table. At nume, a GPhC-registered Independent Prescriber reviews your consultation (your health history, your current medicines, your goals) and recommends the medicine and starting dose that fits your picture. Your treatment arrives the next working day in a plain DPD-tracked box: for Mounjaro, a pre-filled KwikPen ready to use; for Wegovy, a pre-filled injection pen. No subscription locks you in, and if anything changes, our clinical team is available seven days a week.

Ozempic is also semaglutide, but it is licensed for type 2 diabetes, not weight management. It should not be sought or presented as a weight-loss treatment, for a clear explanation of that distinction, our page on how Mounjaro differs from Ozempic sets out the licensing position plainly. If cost is part of your thinking, a comparison of Mounjaro and Wegovy pricing covers what affects the price of each. And if you're ready to talk through which option fits you, start your free consultation with our clinical team today.