Mounjaro vs Wegovy in clinical trials: what the evidence really shows

The myth: the trials never compared them directly, so we can't know which works better

This was a reasonable position until early 2025. For years, clinicians had only indirect comparisons, SURMOUNT-1 for tirzepatide and STEP 1 for semaglutide, both placebo-controlled but run in different populations, at different times, with different protocols. Indirect comparisons are useful, but they carry real uncertainty, which is why NICE noted only that indirect evidence favoured tirzepatide when it recommended Mounjaro in TA1026.

SURMOUNT-5 changed that. It was an open-label, randomised, head-to-head trial, 751 adults with obesity and no type 2 diabetes, followed over 72 weeks, directly comparing tirzepatide (up to 15 mg) against semaglutide 2.4 mg. For the first time, both medicines were tested in the same participants, at the same time, under the same conditions. The result: tirzepatide produced meaningfully greater average weight loss, and if you're weighing up whether Wegovy or Mounjaro is the right fit for you, this trial is the most direct evidence available. The comparison is no longer purely indirect, and the data is now part of the clinical conversation whenever a prescriber is choosing between the two.

What the myth gets right, though, is that a single trial never settles everything. Average results describe populations, not individuals. Our clinical team will tell you the same thing.

What SURMOUNT-5 and the earlier trials actually reported

The table below draws on the three most relevant trials. Numbers describe trial averages, not individual predictions, and cite the published evidence directly.

A few things are worth keeping in mind. SURMOUNT-1 and STEP 1 were not designed to be compared to each other, and their populations differed in ways that matter. SURMOUNT-5 is the cleanest evidence we have, and you can read a fuller breakdown of its methodology and findings on our SURMOUNT-5 trial page.

There is also the question of higher-dose semaglutide. The MHRA approved a dedicated single-dose 7.2 mg Wegovy pen in April 2026, and early data at that dose reported average weight loss of around 20.7% over 72 weeks, which narrows the gap with tirzepatide considerably. If you want to go deeper on what that narrowing gap means in practice, our page looking at which of the two medicines is better works through the evidence alongside the 7.2 mg data. Head-to-head data at 7.2 mg does not yet exist in the published literature, so that comparison remains indirect for now.

What the trials don't settle, and where your consultation comes in

Clinical trial averages are a starting point, not a verdict for any one person. Several things the trials can't answer for you: how your body will respond to a dual GIP and GLP-1 agonist versus a single GLP-1 agonist; which side-effect profile you'll tolerate better; whether the injection format matters to you, or whether you'd prefer an oral option. Both medicines have a GI-led side-effect profile (nausea, indigestion, changes in bowel habit) typically most noticeable after starting or after a dose step, then settling for most people. The NHS medicines information for tirzepatide covers this in plain language if you want to read the specifics.

Eligibility also differs. Tirzepatide is licensed for adults with a BMI of 30 or above, or 27 and above with at least one weight-related condition. NICE's TA1026 recommendations for NHS access have their own criteria. Semaglutide's NICE recommendation under TA875 is restricted to a maximum of two years within a specialist weight-management service, a meaningful practical difference on the NHS, though not for private prescribing. You can explore the broader eligibility picture on our weight-loss treatment overview.

One practical detail patients often raise: keeping a once-weekly pen in the fridge door becomes routine quickly, but if travel or storage is a concern, it's worth discussing at your consultation, both medicines have limited room-temperature windows set out in their patient information leaflets, and your prescriber can talk you through the implications. For a fuller side-by-side look at both medicines beyond the trial data, our Wegovy vs Mounjaro page covers mechanism, dosing schedules and licensing in more depth. There's also a dedicated page on how their costs compare if that's a factor in your thinking, and if semaglutide's role sits within a wider context of injectable weight-loss treatments, our Mounjaro vs Ozempic comparison explains how the two semaglutide formulations differ from tirzepatide across dosing, licensing and intended use.

The trial evidence gives you a map. Which medicine is the right route is a clinical decision, one our prescribers make with you, not for you. Speak to our prescribers through a free consultation, reviewed the same day.