Mounjaro sau Wegovy: comparing two licensed UK weight-loss injections

Step 1 — understand how each medicine works

Mounjaro activates two gut-hormone receptors at once: GIP and GLP-1. Both pathways are involved in appetite regulation and blood-sugar control, and combining them appears to produce a stronger signal for reduced hunger and slower gastric emptying than stimulating either alone. This dual action is what sets Mounjaro apart as the only medicine of its kind currently licensed in the UK for weight management, as confirmed in the NHS tirzepatide medicines page.

Wegovy works through the GLP-1 receptor only. GLP-1 receptor agonists are well-established, semaglutide has been studied extensively over many years across both diabetes and weight-management trials. The mechanism is similar in effect (reduced appetite, delayed stomach emptying) but involves a single pathway rather than two.

In practical terms, both injections are given once a week, into the abdomen, thigh or upper arm, on a rotating schedule. Mounjaro comes as a pre-filled KwikPen; Wegovy uses a pre-filled pen device. It is also worth knowing that tirzepatide and semaglutide are available in tablet form, which may suit people who prefer not to inject. Either pen fits neatly in the fridge door alongside everyday items, the storage requirement for both is 2–8°C, with a short period at room temperature permitted when needed; your Patient Information Leaflet carries the exact window.

Step 2, look at the clinical evidence side by side

The table below summarises key figures from the headline trials for each medicine. Both studies involved adults with obesity and no type 2 diabetes, measured weight loss over roughly 70 weeks, and compared active treatment against placebo alongside lifestyle changes.

The SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, compared tirzepatide directly with semaglutide 2.4mg over 72 weeks in adults with obesity. Tirzepatide produced a greater average reduction in body weight. That said, a newer 7.2mg Wegovy dose (approved by the MHRA in January 2026 and available as a dedicated single-dose pen from April 2026) reported around 20.7% average weight loss, narrowing the gap considerably. Which trial figure is relevant to you depends on the dose you reach, which is always a prescriber's decision. You can read NICE's appraisal of tirzepatide (TA1026) and their earlier semaglutide appraisal (TA875) for the full clinical context.

Step 3, consider eligibility and practical differences

The licensed eligibility criteria for both medicines are similar: adults with a BMI of 30 or above, or 27 or above with at least one weight-related condition such as high blood pressure, high cholesterol, or obstructive sleep apnoea. Lower BMI thresholds apply for certain ethnic backgrounds under UK guidance. Neither medicine is licensed for use in people under 18, during pregnancy, while breastfeeding, or when actively trying to conceive.

There are a few practical distinctions worth knowing. For Mounjaro specifically, women taking the oral contraceptive pill should use an additional non-oral method of contraception for the first four weeks of treatment and for four weeks after each dose increase, because tirzepatide may reduce pill absorption; NHS guidance does not identify the same concern for semaglutide. If you are on HRT, NHS England advises discussing transdermal options with your doctor while on tirzepatide. These are exactly the kinds of details a thorough prescriber assessment will surface, if you're curious how our prescribers approach this, the clinical team profile gives some background.

Thinking about switching from one to the other? The considerations involved are different depending on direction of travel. Our page on moving from Mounjaro to Wegovy covers that pathway, and switching from semaglutide to tirzepatide covers the reverse. If cost is part of your thinking, a straightforward breakdown is available on our Mounjaro and Wegovy price comparison page.

Step 4, how a prescriber makes the decision with you

No comparison table settles this question. A prescriber looks at your full medical history: current medicines, past conditions, whether you have type 2 diabetes, cardiovascular risk factors, previous treatment attempts, and your own preferences about injection devices and routine. Both Mounjaro and Wegovy require a valid prescription following that assessment, they cannot legally be supplied otherwise.

At nume, a GPhC-registered Independent Prescriber reads every consultation personally on the day it arrives. There is no algorithm making the call. If you are already on one of these treatments elsewhere and are thinking about switching, our guide to switching between treatments explains how that works in practice, including what evidence we ask transfer patients to provide.

Further comparisons (including a look at how Ozempic fits into this picture) are covered on our Ozempic vs Mounjaro page, though it is worth noting there that Ozempic is licensed for type 2 diabetes, not weight loss; it is not the right comparison if weight management is your goal. A full overview of our approach is on the weight-loss treatment overview. Which medicine suits you is a clinical decision our prescribers make with you, not one we make here.