Comparing weight loss injections available in the UK

You've seen the adverts and read the headlines, here's what the trial data actually shows

Picture this: you've spent an evening reading about weight loss injections and you've come away with a mix of impressive statistics, conflicting articles and a nagging sense that no one is quite giving you the straight version. That's a reasonable place to be. Here it is plainly.

Mounjaro (tirzepatide, made by Eli Lilly) works by activating two receptors (GIP and GLP-1) involved in appetite regulation and blood-sugar control. Wegovy (semaglutide, made by Novo Nordisk) activates one, the GLP-1 receptor. Both slow gastric emptying and reduce appetite; the dual-action mechanism in tirzepatide appears to produce larger average losses in head-to-head testing.

In the SURMOUNT-1 clinical trial (72 weeks, 2,539 adults with obesity and no diabetes), participants taking tirzepatide at the highest dose lost around 20–21% of body weight on average. In the STEP 1 trial of semaglutide 2.4mg (68 weeks, similar population), the average was around 15%, as published in the New England Journal of Medicine. In SURMOUNT-5, a 72-week open-label trial published in 2025 that put tirzepatide and semaglutide 2.4mg head-to-head directly, tirzepatide produced the greater average reduction. One important caveat: Novo Nordisk's 7.2mg semaglutide dose, approved by the MHRA in early 2026, reported around 20.7% average loss over 72 weeks in its supporting trials, which closes much of the gap with tirzepatide at its highest dose.

Clinical trial averages describe populations, not individuals. If you want to see how these medicines stack up across the full range of doses and outcomes, our page walking through weight loss injections compared sets out the evidence in plain terms. The numbers are a starting point, not a promise.

Eligibility isn't identical for both, and that matters when you're choosing

Both injections are licensed for adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition such as high blood pressure, high cholesterol, obstructive sleep apnoea or type 2 diabetes. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. So far, so similar.

The differences surface when you look at what NICE recommends for NHS access. NICE's appraisal of tirzepatide (TA1026) recommends it for adults with a BMI of 35 or above plus at least one weight-related comorbidity, with NHS rollout happening in cohorts through 2025 and 2026. NICE's guidance on semaglutide (TA875) recommends it only within specialist weight management services, for a maximum of two years, with similar BMI thresholds. Neither route is quick for most people; NHS waiting lists for specialist services can stretch considerably.

For a private weight loss injection, the licensed eligibility criteria in the SmPCs apply rather than the NHS-specific NICE thresholds. That means a BMI of 30 or above, or 27 or above with a qualifying condition, is the relevant starting point, subject to the full clinical picture a prescriber reviews.

One practical difference worth knowing: women using oral contraceptives who start tirzepatide should add a barrier method for the first four weeks of treatment and for four weeks after each dose increase, because tirzepatide can affect how the pill is absorbed. NHS guidance notes there is no equivalent evidence of reduced pill effectiveness with semaglutide. It's a small but specific consideration to raise during any consultation.

Dose schedules, tolerability and the detail that gets left out of most comparisons

Both injections are once-weekly, subcutaneous, and come as pre-filled pens. Mounjaro is a KwikPen where each pen contains four weekly doses; Wegovy comes as individual pre-filled pens. Injection sites are the same for both: abdomen, thigh or upper arm, rotated to avoid using the same spot twice.

Mounjaro starts at 2.5mg. That first dose exists purely to help your body adjust, it's a tolerability starting point, not where the treatment effect sits. Doses are typically increased in 4-week steps by the prescriber, up to a maximum of 15mg. Wegovy follows a similar stepped approach, starting at 0.25mg and escalating up to 2.4mg (or 7.2mg under the newer approval). The graduated schedules for both medicines exist to reduce early gastrointestinal side effects: nausea, diarrhoea, constipation and indigestion are common in the first weeks and tend to settle as the body adapts.

Both pens require refrigerated storage between 2°C and 8°C, with a limited window at room temperature, the exact duration is stated in the Patient Information Leaflet for each medicine, and that's the right place to check it rather than any secondary source. Storage is worth thinking about before you travel.

For a side-by-side breakdown, our weight loss injections comparison chart sets the key figures out clearly. If you want to dig further into how individual providers approach clinical oversight and aftercare, the criteria for choosing the right provider is worth reading before you commit to anything.

What this comparison can't tell you, and what can

A weight loss injection comparison like this one can tell you about averages, mechanisms and licensing, and if you'd like to compare weight loss injections across a broader set of practical factors, including cost, availability and titration schedules, that dedicated page covers it in more detail. It can't tell you which medicine your prescriber will consider most suitable, whether your current medicines interact, or how your body will respond. Those things come from a proper clinical assessment.

Both Mounjaro and Wegovy are prescription-only medicines. The legal route to either in the UK is a prescription issued after a clinician has reviewed your health history, current medications, BMI, and any conditions that might affect suitability. Buying either injection from a seller offering it without a clinical assessment is not a short-cut, it's a serious safety risk. The MHRA has warned repeatedly about counterfeit weight loss pens circulating online; you can check whether any pharmacy you're considering is GPhC-registered at the GPhC register.

At nume, every consultation is read the same day by a GPhC-registered Independent Prescriber, a real clinician, not automated software. If you are based in the Midlands and want to understand local access options, our guide to weight loss injections in Stoke-on-Trent explains what's available and how the process works in that area. If you'd like to explore which injection might be appropriate for you, start a free consultation with our clinical team. There's no obligation, and the consultation itself costs nothing. It's also worth browsing our treatment overview if you want context on how the medicines fit into a broader approach to weight management before you take the next step.