What weight loss injections are there in the UK?

What the trial data tells us about these two injections

When NICE evaluated both medicines, it drew on large randomised controlled trials. SURMOUNT-1, published in the New England Journal of Medicine, followed 2,539 adults with obesity over 72 weeks. Participants on tirzepatide 15mg lost an average of around 20–21% of their body weight alongside a reduced-calorie diet and increased activity. The STEP 1 trial ran semaglutide 2.4mg against placebo over 68 weeks and found an average weight reduction of around 15%. In the more recent SURMOUNT-5 trial (72 weeks, 751 adults, published in NEJM 2025), tirzepatide produced greater average weight loss than semaglutide 2.4mg head-to-head.

These figures come from controlled trial conditions. Real-world results vary, and no injection guarantees a specific outcome for any individual. That caveat matters, but the direction of the evidence is consistent enough that NICE recommended both medicines for NHS use, at different criteria and within different service settings. What trial data cannot tell you is which injection suits your health profile. That is a clinical decision, not a comparison table.

One practical note: in the SURMOUNT-5 trial, Wegovy's newer 7.2mg dose was not the comparator, semaglutide 2.4mg was. The MHRA approved a dedicated 7.2mg Wegovy pen in April 2026, and early data suggests average weight loss at that dose approaches tirzepatide 15mg results. The picture is still developing.

How Mounjaro and Wegovy differ in the way they work

Both injections slow gastric emptying and reduce appetite, but they activate different receptors. Wegovy is a GLP-1 receptor agonist, mimicking a gut hormone that signals fullness and regulates blood sugar. Mounjaro adds a second pathway: it also activates the GIP receptor, a complementary gut hormone involved in energy regulation. This dual mechanism is why tirzepatide is the only medicine of its type currently licensed for weight management in the UK.

In practice, both are once-weekly subcutaneous injections using pre-filled pens. Treatment begins at a low starting dose to let the body adjust (the first pen's job is largely tolerability rather than maximum effect) and the dose is increased step by step by the prescriber over several months. Injection sites rotate between the abdomen, thigh and upper arm. Storage is in the fridge at 2–8°C; each pen comes with instructions on the limited window it can be kept at room temperature, and the Patient Information Leaflet is the definitive reference for that detail.

People sometimes ask about Saxenda (liraglutide), an older daily injection that is UK-licensed for weight management but less widely used since once-weekly options became available. Ozempic is also semaglutide but is licensed for type 2 diabetes, not weight loss, it is not a weight-loss injection despite often appearing in searches. If you are curious about what the weight loss injections actually are and how they compare, that page covers the landscape alongside our weight-loss injections overview.

Who these injections are licensed for, and what eligibility actually means

Both medicines 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, type 2 diabetes, obstructive sleep apnoea or cardiovascular disease. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. BMI alone is not a green light: a prescriber weighs the whole clinical picture, including other medicines being taken, relevant medical history and individual circumstances.

NICE's recommendation for tirzepatide (TA1026, updated September 2025) sets NHS eligibility at BMI 35 or above with at least one qualifying comorbidity, within a phased rollout. NICE's recommendation for semaglutide (TA875) requires use within a specialist weight management service for a maximum of two years. Private routes through a regulated online pharmacy follow the licensed criteria from the SmPCs rather than the NHS service criteria, making them accessible to people who do not yet qualify under the phased NHS thresholds or who prefer not to wait.

Injections are not licensed for under-18s. Neither is recommended during pregnancy, while breastfeeding, or when trying to conceive. Women on oral contraceptives starting tirzepatide should add a barrier method for the first four weeks and for four weeks after each dose increase, because the pill's absorption may be reduced during that period, NHS England's weight-management injections guidance covers this clearly.

Side effects, safety and what ongoing care looks like

The most commonly reported side effects with both injections are gastrointestinal: nausea, diarrhoea, constipation, indigestion and burping appear most often. They tend to be most noticeable in the first days after starting or after a dose increase, and for many people they settle within a couple of weeks. Fatigue, headache and mild dizziness are also reported. Injection-site reactions (redness, swelling or bruising) can occur and usually resolve quickly with proper technique and site rotation.

Pancreatitis is an infrequent but serious risk. The MHRA issued a Drug Safety Update on this in January 2026, flagging that severe, persistent stomach pain (particularly if it spreads toward the back) warrants urgent medical assessment, whether or not vomiting is present. Suspected side effects from any medicine can be reported directly to the MHRA via the Yellow Card scheme.

Proper aftercare matters more than many people realise before they start. Dose decisions, managing early side effects and knowing when to pause or stop are all clinician territory, not something to work out from forums. Tell your surgical team and anaesthetist about any GLP-1 injection before a procedure, NHS England advises this specifically. A sharps container for used pens and alcohol swabs for the injection site are the practical side of ongoing treatment that is easy to overlook.

When you order through a service like nume, your pen arrives via DPD the next working day in plain, unbranded packaging, tracked to your door. That is one small thing; the more important one is that a GPhC-registered prescriber, not automated software, reviews your consultation before anything is dispensed. If you want to understand what types of weight loss injections are available and how they differ, including what the three main weight loss injections are, those pages go into further detail, and if you want to explore what other weight loss injections exist beyond the most widely discussed options, that is covered there too. For a closer look at what type of weight loss injections there are and which might suit your circumstances, our clinical team page explains who is involved in the review process. And if you have questions before committing to anything, our FAQs cover the ones we hear most often.