The Difference Between Weight Loss Injections Available in the UK

Picture this: you've found two injections, both once-weekly, both in a pre-filled pen, so what's different?

On the surface they look similar. Same routine (one injection per week, into the abdomen, thigh or upper arm), same broad goal (supporting significant weight reduction alongside a lower-calorie diet and more activity), similar gradual dose-increase schedule that lets your body settle in. But the medicine inside each pen works differently, and that matters clinically.

Mounjaro contains tirzepatide, made by Eli Lilly. It activates two gut-hormone receptors simultaneously: GIP and GLP-1. That dual action is why it's sometimes described as a step beyond the original wave of weight-loss injections. Wegovy contains semaglutide, made by Novo Nordisk, and acts on one receptor, GLP-1. That single-pathway approach still produces substantial weight loss for many people; semaglutide was, until recently, the most studied weight-loss medicine available. Both slow gastric emptying and reduce appetite through different but overlapping mechanisms. If you want to understand the different kinds of weight loss injections available and how they compare, that context helps explain why a prescriber would choose one over the other for a given patient.

One practical note: with tirzepatide, women using the oral contraceptive pill should add a barrier method for the first four weeks of treatment and for four weeks after each dose increase, because absorption of the pill may be reduced. There's no equivalent evidence of this effect with semaglutide. If that's relevant to you, mention it during your consultation. Our FAQs cover this and other common questions in more detail.

What the clinical trial evidence actually shows

Numbers help here. The table below sets out the headline figures from the two key trials.

Those figures come from trials in thousands of adults with obesity, published in the New England Journal of Medicine and reviewed by NICE. The SURMOUNT-1 trial alone enrolled 2,539 participants. At the highest dose studied, tirzepatide produced roughly 20–21% average weight reduction; at the standard 2.4mg maintenance dose, semaglutide produced around 15%. The head-to-head SURMOUNT-5 trial, published in the NEJM in 2025, found tirzepatide achieved greater average loss.

That gap narrows considerably at Wegovy's newer 7.2mg dose, approved by the MHRA in January 2026 and available in a dedicated single-dose pen approved in April 2026. Whether that dose suits a particular patient depends on their clinical history and tolerance.

These are averages from controlled trials, not a guarantee for any individual. A prescriber weighs your whole picture, not a percentage, when recommending a starting point. NICE's appraisal of tirzepatide is available at NICE TA1026 for anyone who wants to read the full evidence review.

Who each injection is licensed for, and who Ozempic is not for

Both Mounjaro and Wegovy are licensed in the UK for weight management in adults with a BMI of 30 or above, or 27 or above alongside at least one weight-related health condition (such as high blood pressure, type 2 diabetes, or high cholesterol). Lower thresholds apply for some ethnic backgrounds under UK guidance. Whether you meet the criteria is something a prescriber confirms during your assessment, the BMI figure is the starting point, not the whole story.

Ozempic is also semaglutide, and this is where people often get confused. Ozempic is licensed in the UK for type 2 diabetes, not weight management. Prescribing it for weight loss would be off-label. The MHRA is clear on this, and you can read their public guidance on the MHRA's Yellow Card site alongside reporting tools for suspect sellers. Rybelsus (oral semaglutide tablets at 3, 7 and 14mg) is also a diabetes medicine, not a weight-loss one. If you're searching because you've heard about these names, the honest answer is that the licensed weight-loss versions are Mounjaro and Wegovy.

Both Mounjaro and Wegovy carry a Black Triangle (▼) designation from the MHRA, meaning they're subject to additional monitoring while post-market safety data continues to build. That's standard for newer medicines and doesn't mean they're unsafe; it means regulators are actively collecting real-world evidence. The NHS's tirzepatide page and the equivalent semaglutide page both carry patient-level information on side effects and precautions worth reading before you start either medicine.

How to decide which injection is right for you, and what comes next

The honest answer is that you probably can't decide from a webpage, and you shouldn't have to. The right injection depends on your BMI, your medical history, any medicines you already take, your preference for injection versus tablet, and clinical judgement that a prescriber applies to all of it together. If you're still weighing up the options, it can help to read about the different weight loss injections that are currently available, so you arrive at your consultation with a clearer starting point.

At nume, that conversation starts with a free consultation. A prescriber reads your medical details personally, our clinical team includes GPhC-registered Independent Prescribers who review every case themselves. If you're a good fit for treatment, the same-day dispatch means your pen arrives with DPD tracking the next working day, in a plain box with nothing on the outside to say what's inside. If you've been reading about weight loss injections and want to understand how they work before taking the next step, the consultation is the right place to bring those questions.

Which injection suits you is a clinical decision our prescribers make with you, not something we decide in advance. To help you prepare, it's worth reading about the difference in weight loss injections so you can go into your consultation with a clearer sense of what to ask. Start your free consultation and find out where the evidence and your own situation point.