How Wegovy Works Compared to Mounjaro: What the Trial Evidence Shows

What the head-to-head trial actually found

The most direct evidence comes from SURMOUNT-5, a 72-week open-label trial published in the New England Journal of Medicine in 2025. It randomised 751 adults living with obesity but without type 2 diabetes to receive either tirzepatide or semaglutide 2.4mg (the standard maintenance dose of Wegovy at the time) and tracked average weight change over the course of the study. Tirzepatide produced greater average weight reduction than semaglutide 2.4mg. That result aligned with what NICE observed when evaluating the indirect evidence before publishing its tirzepatide appraisal (NICE TA1026, December 2024).

For context on what each medicine achieves independently: in SURMOUNT-1 (also 72 weeks), tirzepatide at 15mg produced average body-weight reductions of around 20–21%. In the STEP 1 trial, semaglutide 2.4mg produced an average reduction of around 15% over 68 weeks (NEJM, STEP 1). Neither figure is a promise, individual results vary, and both trials measured outcomes in specific clinical populations with intensive lifestyle support alongside the medicine.

One development worth noting: the MHRA approved a higher 7.2mg semaglutide dose in January 2026, and a dedicated single-dose 7.2mg Wegovy pen in April 2026. Early data at this higher dose reported around 20.7% average weight loss over 72 weeks, narrowing the gap with tirzepatide 15mg considerably. That changes how the two medicines compare, and it is one reason the conversation about which to choose belongs with a prescriber who can weigh current evidence against your own circumstances. If you want to understand how the two medicines stack up across doses and trial data, our page on Wegovy compared to Mounjaro brings that evidence together in one place. You can explore the effectiveness evidence in more detail if that is the specific question you are trying to answer.

Different receptors, different effects on appetite and blood sugar

Semaglutide, the active ingredient in Wegovy, binds to GLP-1 receptors. GLP-1 is a gut hormone released after eating; stimulating its receptor slows gastric emptying, reduces appetite signals in the brain, and encourages the pancreas to release insulin in a glucose-dependent way. The result is less hunger, earlier fullness, and a modest improvement in blood sugar regulation.

Tirzepatide, the active ingredient in Mounjaro, does all of that and also activates GIP receptors. GIP is a second gut hormone with overlapping but distinct roles in fat metabolism, energy use and satiety signalling. Activating both pathways at once appears to produce a more pronounced appetite effect for most people, which is the leading hypothesis for why weight loss is greater on average. Mounjaro is the only dual GIP and GLP-1 receptor agonist licensed for weight management in the UK. For a deeper look at the ingredient-level differences, the ingredient comparison page covers the pharmacology in more detail.

The practical upshot: both medicines slow how quickly your stomach empties, both reduce appetite, and both are given as once-weekly subcutaneous injections. The difference is in degree, not in kind. Side-effect profiles are also broadly similar, nausea, constipation, and other gastrointestinal symptoms are common early on with both, typically settling over the first few weeks. The NHS tirzepatide page and the NHS semaglutide page both set out the full side-effect profiles in plain terms.

How they compare at a glance

A note on Ozempic: it contains the same active ingredient as Wegovy (semaglutide) but is licensed for type 2 diabetes, not weight management. It should not be confused with Wegovy, and it is not an option for weight loss via a legitimate UK prescription. The Mounjaro versus Ozempic page explains that distinction clearly.

Eligibility, storage, and which suits you

Both medicines share similar private eligibility criteria: adults with a BMI of 30 or above, or 27 or above with at least one weight-related condition such as high blood pressure, type 2 diabetes or high cholesterol. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. A prescriber considers your full medical picture (other medicines, existing conditions, previous treatment) before recommending one over the other. Those factors matter as much as the headline trial figures.

On the practical side: Mounjaro pens are kept in the fridge at home, typically settling into a routine alongside other fridge regulars, a pen that goes back between appointments without much thought. The Wegovy injection also requires refrigeration per the patient information leaflet. Neither is particularly complicated once the routine is established, but your prescriber will walk through storage and administration at the point of prescription.

The question of which medicine performs better for a specific individual cannot be answered from population averages alone. For people weighing up the two in detail, the full Wegovy versus Mounjaro comparison and the cost comparison between tirzepatide and semaglutide are worth reading alongside this page. Which suits you is a clinical decision our prescribers make with you, based on your health history and your goals, not on which medicine trends highest in a search.