Wegovy vs Mounjaro: Which Weight-Loss Medicine Is Right for You?

The core difference: one hormone pathway vs two

Wegovy contains semaglutide, a GLP-1 receptor agonist. It mimics one gut hormone involved in appetite regulation and slows gastric emptying, which reduces how hungry you feel between meals. Mounjaro contains tirzepatide, the only dual GIP and GLP-1 receptor agonist licensed for weight loss in the UK, it activates two gut-hormone pathways simultaneously. Whether that biological difference translates into a meaningful clinical difference for you is exactly what the head-to-head trial data addresses.

Both medicines are injected once a week using a pre-filled pen. Both are Black Triangle (▼) medicines under additional MHRA monitoring. Both are licensed for adults with a BMI of 30 or above, or 27 and above alongside at least one weight-related condition such as high blood pressure, type 2 diabetes or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. None of that licensing detail tells you which one is right for you, that's what the prescriber works out.

A question our prescribers hear most weeks: 'I've heard Mounjaro is stronger, should I just go straight to that?' The honest answer is that strength on average doesn't equal right for a specific person. Tolerability, your medical history, other medicines you take, and what you've tried before all shape the answer more than trial headlines do.

For a fuller look at switching between treatments, the moving from Mounjaro to Wegovy page covers what that process looks like in practice.

What the trial data shows, side by side

The most rigorous comparison available is SURMOUNT-5, an open-label head-to-head trial published in the New England Journal of Medicine in 2025. It ran for 72 weeks with 751 adults living with obesity but without diabetes. Tirzepatide produced greater average weight reduction than semaglutide 2.4mg across that period. NICE's appraisal of tirzepatide noted that indirect comparisons also favour tirzepatide over semaglutide 2.4mg, while acknowledging the limitations of cross-trial inference.

Looking at the individual trials: SURMOUNT-1 (72 weeks, tirzepatide 15mg) reported around 20–21% average body-weight reduction. STEP 1 (68 weeks, semaglutide 2.4mg) reported around 15% average reduction. The higher Wegovy 7.2mg dose, approved by the MHRA in January 2026, produced around 20.7% average loss over 72 weeks in trials, narrowing the gap considerably. These are group averages from controlled conditions; individual results vary.

For a detailed price breakdown of both options in the UK private market, the Mounjaro vs Wegovy UK price comparison page covers what you'd actually pay and what's included. If you're weighing up which treatment fits your situation, our Wegovy v Mounjaro guide walks through the broader evidence picture in detail. The Wegovy vs Mounjaro overview page covers the broader evidence picture if you want more depth on the clinical data.

Side-effect profiles and tolerability: more similar than different

Both medicines share a GLP-1 mechanism, so their side-effect profiles overlap considerably. The most commonly reported effects are gastrointestinal: nausea, loose stools, constipation, reflux, bloating, and fatigue. These tend to be most noticeable after a new dose starts, then ease over days to a couple of weeks as your body adjusts. Injection-site reactions (mild redness or discomfort) can also occur with either pen.

Serious but less common risks that apply to both include acute pancreatitis. The MHRA issued a Drug Safety Update in January 2026 specifically highlighting this risk for GLP-1 medicines: severe stomach pain that radiates to the back, with or without vomiting, warrants urgent medical attention. Gallbladder problems, dehydration from prolonged GI illness, and allergic reactions are also documented for both.

One practical distinction: for women using oral contraceptives, Mounjaro (tirzepatide) can reduce pill absorption during the first four weeks of treatment and for four weeks after each dose increase. Adding a non-oral contraceptive method during those windows is advised. Current evidence does not show the same effect with semaglutide, though the guidance is to discuss contraception with your prescriber before starting either medicine. The NHS England guidance on weight management injections covers both the contraception and HRT considerations in full.

Neither medicine is recommended during pregnancy, while breastfeeding, or when trying to conceive. Both are unlicensed in people under 18. Your medical history (including any history of pancreatitis, thyroid conditions, or certain gastrointestinal disorders) determines whether either medicine is suitable at all, which is why the prescriber review matters before any prescription is written.

How the clinical decision actually gets made

The question of Wegovy or Mounjaro isn't one you need to resolve before your consultation. Our prescribers review your full health picture (not just your BMI) and factor in your medical history, any current medicines, prior weight-loss treatment if relevant, and your own preferences. For people who want a structured comparison before their appointment, our Mounjaro va Wegovy page sets out the key clinical differences side by side. For people transferring from one medicine to the other, the clinical picture is reviewed fresh; the process of moving from Mounjaro to Wegovy is a separate clinical assessment, not an automatic switch.

Both SURMOUNT-5 and the NICE appraisal of tirzepatide (TA1026) inform our prescribers' thinking. So does NICE's guidance on semaglutide. But guidelines describe populations; a prescriber's job is to apply them to one person at a time. Which of these medicines suits you is a clinical decision our prescribers make with you, not one the data alone can settle. If you'd like to understand how the consultation works, the weight-loss treatment overview explains the full process, and the team behind the clinical decisions is introduced on the clinical team page.