Weight Loss on Wegovy vs Mounjaro: What the Evidence Actually Shows

Picture this: you've been offered both options and want to know which one actually shifts more weight

A question our prescribers hear most weeks goes roughly like this: "I know they both work — but which one works more?" It's a fair question, and the trial data gives a reasonably clear answer at the population level, even if that answer doesn't tell you what will happen to you specifically.

Wegovy's phase-3 programme, the STEP trials, enrolled thousands of adults with obesity or overweight. The STEP 1 trial, published in the New England Journal of Medicine, reported an average body-weight reduction of around 15% over 68 weeks at the 2.4mg maintenance dose, compared to roughly 2.4% with placebo, in adults without diabetes. That's a substantial result.

Mounjaro's SURMOUNT programme produced larger average figures. In SURMOUNT-1 (2,539 participants, 72 weeks), adults on the highest 15mg dose saw an average weight reduction of around 20–21%. Across some analyses the figure reached approximately 22.5%. Both programmes ran alongside reduced-calorie diet and increased physical activity, the medicines don't operate in isolation from lifestyle.

Population averages, though, describe a distribution. Some participants lost considerably more than the mean; others less. The range on both medicines is wide. That context matters when you're setting your own expectations. You can read more about the SURMOUNT-5 trial results, which added a direct head-to-head layer to this comparison.

The head-to-head trial: what SURMOUNT-5 added to the picture

For years the Wegovy and Mounjaro evidence bases ran in parallel, not against each other. SURMOUNT-5, published in the New England Journal of Medicine in 2025, was the first large randomised head-to-head: 751 adults with obesity and no diabetes, followed for 72 weeks, comparing tirzepatide (up to 15mg) directly with semaglutide 2.4mg.

Tirzepatide produced greater average weight reduction than semaglutide 2.4mg in that trial. NICE's appraisal of tirzepatide (TA1026) noted in its committee discussion that indirect comparisons also favour tirzepatide, while acknowledging that the comparison is imperfect given differences in trial populations and design.

One important caveat: the semaglutide arm used the 2.4mg dose. A higher 7.2mg Wegovy dose (approved by the MHRA in January 2026 and available as a dedicated single-dose pen from April 2026) was not part of SURMOUNT-5. Trials at 7.2mg have reported average weight loss of around 20.7% over 72 weeks, which narrows the gap with tirzepatide's top dose considerably. The practical question of Wegovy not working as well as Mounjaro looks different in that light.

The headline, fairly stated: at the doses tested head-to-head, tirzepatide produced larger average reductions. At 7.2mg, semaglutide closes much of that gap. Neither medicine has a universally superior profile, tolerability, personal history and clinical factors all feed into which is the right starting point for a given person.

Mechanism, eligibility and side-effect profile: the three clinical differences that matter

Mounjaro is the only dual GIP and GLP-1 receptor agonist licensed for weight management in the UK. It activates two gut-hormone pathways involved in appetite regulation and blood-sugar control, slowing gastric emptying and increasing satiety. Wegovy acts on the GLP-1 receptor alone. Whether the dual mechanism explains the larger average weight reductions in trials is an active area of research; what's established is that both slow how quickly the stomach empties and reduce appetite, producing a calorie deficit most people wouldn't sustain through willpower alone.

Eligibility under the licensed criteria is broadly similar: adults with a BMI of 30 or above, or 27 and above with at least one weight-related condition such as high blood pressure, type 2 diabetes, dyslipidaemia or obstructive sleep apnoea. NICE recommends tirzepatide (TA1026) for NHS use from a BMI of 35 with one or more comorbidities, and recommends semaglutide (TA875) within specialist weight management services for a maximum of two years. Both come with ethnic-background thresholds approximately 2.5 kg/m² lower under UK guidance.

Side-effect profiles are similar too, dominated by gastrointestinal effects: nausea, loose stools, constipation, reflux and fatigue are common to both, most noticeable when starting or stepping up a dose. For tirzepatide specifically, women taking the oral contraceptive pill should use an additional non-oral method for the first four weeks of treatment and for four weeks after each dose increase, because absorption may be reduced. The NHS patient information for tirzepatide sets this out in detail. Both medicines carry a Black Triangle (▼) status, meaning the MHRA continues to monitor their safety data closely.

Switching, stopping and the longer view

Some people start on one medicine and move to the other, either because results plateau, tolerability becomes an issue, or the prescriber judges that a different mechanism or dose trajectory is more appropriate. The evidence on switching is still developing. If you've tried semaglutide and found results disappointing, the clinical picture for switching direction is worth understanding before drawing conclusions.

What both medicines share is a dependency that matters: when treatment stops, appetite typically returns and weight tends to come back over time. The evidence on weight regain after stopping is clear enough that any honest comparison has to include it. These are long-term treatments for a long-term condition, not short courses.

For a side-by-side look at how the two medicines compare across more dimensions, including cost context, the detailed Wegovy vs Mounjaro overview covers the broader picture. If pricing is your primary question, the cost comparison page sets out what private treatment typically involves.

Which medicine suits you is a clinical decision our prescribers make with you, taking into account your health history, any medicines you already take, your tolerance for side effects and your goals. Start your free consultation and a GPhC-registered prescriber will review your answers the same day.