The main types of weight loss injections used in the UK today

How the two licensed injection types actually differ

The starting point worth getting right: the phrases "weight loss injection" and "slimming jab" are used loosely in the media to cover several products, and that looseness causes real confusion. In the UK right now, two injectable medicines hold a licence specifically for weight management in adults. Tirzepatide, sold as Mounjaro by Eli Lilly, and semaglutide, sold as Wegovy by Novo Nordisk. That is the complete list for weight management injections. Saxenda (liraglutide) was previously available but is not dispensed by our weight-loss service, and Ozempic (which is also semaglutide) is licensed for type 2 diabetes rather than weight loss, despite the name often appearing in the same breath as Wegovy.

The pharmacological difference between the two types is real and worth understanding. Semaglutide works on one receptor, GLP-1, which is involved in appetite signalling and slowing the rate at which the stomach empties. Tirzepatide acts on both GLP-1 and a second receptor, GIP, making it the only dual-agonist weight-loss medicine licensed in the UK. Whether that additional pathway translates to meaningfully different outcomes for a given individual is a clinical conversation, not something the mechanism alone settles.

Both types are delivered via a pre-filled pen injected into the abdomen, thigh or upper arm, once a week. Both start at a low tolerability dose and are stepped up over several months by the prescriber. If you'd like to understand each option in more detail before exploring further, our guide to the 3 types of weight loss injections walks through each one clearly. You can also read a fuller breakdown of how weight loss injections work if the mechanism is what you came to understand first.

What the clinical trials found — and what that means in practice

The evidence base for both types is substantial and comes from large, rigorously designed phase 3 trials published in peer-reviewed journals. The STEP 1 trial, published in the New England Journal of Medicine, studied semaglutide 2.4 mg over 68 weeks in adults with obesity and found an average body-weight reduction of around 15% compared with placebo. That result was the basis for Wegovy's UK licence and NICE's recommendation.

Tirzepatide's SURMOUNT-1 trial, also published in the NEJM, randomised 2,539 adults with obesity over 72 weeks. At the 15 mg dose, average body-weight reduction was approximately 20–21%. SURMOUNT-5, a head-to-head trial published in 2025, compared tirzepatide directly against semaglutide 2.4 mg and found tirzepatide produced greater average weight loss over 72 weeks. A note on scale: SURMOUNT-1 alone involved nearly 2,500 participants; the wider tirzepatide programme across SURPASS and SURMOUNT trials included more than 10,000 people in total.

There is a common assumption that a higher percentage reduction automatically means the "better" injection for every patient. It does not. Starting weight, existing health conditions, medicines already being taken, personal tolerance of side effects and practical preferences all shape which type a prescriber would consider appropriate. The trials tell us what happened on average across populations; they do not predict individual response. Our clinical team reviews each consultation individually, precisely because that nuance matters.

Eligibility: who can be prescribed which injection type

Both licensed injection types share a similar eligibility framework drawn from their UK product licences. Adults with a BMI of 30 or above qualify, as do adults with a BMI of 27 to 29.9 who have at least one weight-related condition, for example, high blood pressure, raised cholesterol, obstructive sleep apnoea or prediabetes. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance, reflecting differences in cardiovascular risk at a given BMI.

NICE's appraisal of tirzepatide (TA1026) recommends it for NHS use in adults with a BMI of 35 or above plus at least one weight-related comorbidity, with NHS rollout phased by BMI and number of conditions. NICE's recommendation for semaglutide (TA875) adds a further condition: it should be used for a maximum of two years, within a specialist weight management service. Private prescribing operates to the broader licence criteria, though suitability still requires individual clinical assessment.

Neither injection type is licensed for people under 18, and both are not recommended during pregnancy, while breastfeeding or for those trying to conceive. These are not arbitrary cautions, the prescriber will ask about all of these as part of the consultation. For a detailed look at how the injection types compare on eligibility, that page sets out the licence criteria side by side.

Side effects, safety monitoring and what to watch for

Both injection types share a broadly similar side-effect profile because both slow gastric emptying. Nausea is the most commonly reported effect, particularly in the first few weeks or after a dose increase; vomiting, diarrhoea, constipation and indigestion are also common. For most people these effects settle as the body adjusts, though the timeline varies. Starting at the lowest dose and titrating gradually is the approach specifically designed to reduce this.

More serious but less frequent effects are worth knowing about. In January 2026, the MHRA issued a Drug Safety Update noting acute pancreatitis as a known infrequent risk with GLP-1 medicines. Severe, persistent stomach pain that spreads to the back, with or without vomiting, warrants urgent medical attention. Gallbladder problems, allergic reactions and signs of dehydration from prolonged vomiting are further reasons to seek help promptly. Patients and healthcare professionals can report suspected side effects via the MHRA Yellow Card scheme.

Both medicines carry a Black Triangle (▼) designation, meaning the MHRA continues to collect safety data as real-world use grows. This is standard for newer medicines and does not signal unusual danger, it simply means post-market surveillance is ongoing. Storing your pen correctly matters too: both types require refrigeration between 2–8 °C, with a limited window at room temperature; the Patient Information Leaflet supplied with your pen gives the exact figures. If you keep a sharps container at home, used pens can be disposed of safely without needing to visit a pharmacy.

If you have questions about a specific side effect or want to understand how the consultation process works, our FAQ page covers many of the practical details, including what types of weight loss injections are available, or you can get in touch directly. When you feel ready to take the next step, our prescribers are available to review your information the same day, and if you are based locally you can find out more about accessing weight loss injections in Bradford, or you can start a free consultation here.