Weight loss injections like Mounjaro: what they are, how they differ, and what actually matters

You've seen Mounjaro mentioned everywhere, here's the fuller picture

You're probably here because someone mentioned Mounjaro, or because you've spent an evening reading about weight loss jabs and can't quite work out how they relate to each other. That's understandable. The landscape changed fast. Until a few years ago, the only injectable option for weight management most people had heard of was Saxenda. Now there are two well-publicised alternatives, and the distinctions between them are worth understanding before you speak to a prescriber.

Mounjaro is the brand name for tirzepatide, made by Eli Lilly. It's a dual GIP and GLP-1 receptor agonist, it acts on two separate gut-hormone pathways that regulate appetite, fullness and blood-sugar control. That dual mechanism is what makes it different from every other weight loss injection currently licensed in the UK. Wegovy, which contains semaglutide by Novo Nordisk, targets the GLP-1 receptor only. Both slow stomach emptying and reduce appetite; they just get there by different routes.

One misconception worth clearing up gently: Ozempic also contains semaglutide, but it's licensed in the UK for type 2 diabetes, not weight management. The two are not interchangeable for this purpose, and a UK prescriber won't issue it for weight loss. If you've seen Ozempic discussed in weight-loss forums, that's the reason it doesn't appear as an option here.

If you'd like a grounded overview of what tirzepatide is and how it's used, our guide to Mounjaro weight loss injections covers the mechanism, eligibility and what to expect from treatment in plain terms.

What the trial evidence shows, and what it doesn't promise

The clinical results for both medicines come from large, well-designed randomised trials, and they're worth knowing. In the SURMOUNT-1 trial, involving 2,539 adults with obesity and no diabetes over 72 weeks, participants on tirzepatide 15mg lost an average of around 20–21% of their body weight alongside lifestyle changes, as published in the New England Journal of Medicine. The STEP 1 trial for semaglutide 2.4mg (Wegovy) showed an average reduction of around 15% over 68 weeks.

In SURMOUNT-5, a head-to-head trial published in the NEJM in 2025, tirzepatide produced greater average weight loss than semaglutide 2.4mg over 72 weeks in adults with obesity. Novo Nordisk has since received MHRA approval for a 7.2mg Wegovy dose that, in trials, achieved around 20.7% average loss, narrowing that gap considerably.

These are averages. Some participants lost more; some lost less. Trial conditions (structured diet support, regular check-ins, strict adherence monitoring) differ from the real world. The figures give a credible benchmark, not a personal guarantee. A good prescriber will set expectations accordingly, and how quickly results appear depends on a range of individual factors too.

Both medicines are black triangle (▼) products, meaning the MHRA requires additional monitoring of post-market safety data. That's a standard classification for newer medicines, not a cause for alarm, but it does mean reporting any unexpected side effects matters, the Yellow Card scheme at yellowcard.mhra.gov.uk is how patients do that.

Who these injections are licensed for, and what a prescriber looks at

Mounjaro is licensed for adults with a BMI of 30 or above, or 27 and above with at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. Wegovy carries similar eligibility criteria. Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance, a prescriber will check this during your assessment.

BMI alone isn't the whole picture. Prescribers look at your medical history, any current medicines you take, and conditions that might make these treatments unsuitable. Both medicines are not recommended during pregnancy, breastfeeding or if you're trying to conceive, and neither is licensed for under-18s. Anyone with a history of medullary thyroid carcinoma or certain gastrointestinal conditions will need a detailed conversation before any GLP-1 medicine is considered.

The NICE appraisal of tirzepatide (TA1026) sets NHS eligibility at BMI 35 or above with at least one qualifying comorbidity, with phased rollout criteria, but private prescribing follows the licensed indications from the SmPC, which are broader. If you're based in Yorkshire and trying to understand your options, our page on Mounjaro weight loss injections in Leeds covers both NHS and private access in more detail.

Getting started through a regulated pharmacy, not a social media seller

These are prescription-only medicines. The only legal route to them in the UK is through a registered prescriber and a pharmacy you can verify. The GPhC pharmacy register lists every legally operating pharmacy, checking takes about thirty seconds and it's genuinely the most useful thing you can do before handing over any money online.

The MHRA has issued repeated warnings about counterfeit pens sold through social media and unverified websites. Fake pens have been found to contain insulin instead of the stated medicine, and in October 2025 a site manufacturing counterfeit tirzepatide pens in the UK was raided. Large seizures of fake products have followed. Red flags are consistent: no clinical assessment, prices far below the private market, discount codes. For a prescription medicine, price is genuinely the wrong measure of value. Safe supply, clinical oversight and genuine product are what keep treatment effective.

At nume, every consultation is reviewed the same day by a GPhC-registered Independent Prescriber. Our clinical team reads your answers personally, not software. If you'd like to understand the cost side before starting, our Mounjaro pricing page explains what a legitimate private price includes and why the numbers vary between providers. For anyone who wants to read more about how weight loss injections using tirzepatide work before booking, that's a good place to start, and if you're also trying to find Mounjaro weight loss injections near you, our location guide can help you understand what to look for in a local provider, and when you're ready, starting a free consultation is the right next step.