The Different Weight Loss Injections Licensed in the UK

The myth worth correcting first: these injections are not all the same drug under different brand names

A surprisingly common assumption is that Mounjaro and Wegovy are interchangeable products, just priced differently. They are not. They contain distinct active ingredients (tirzepatide and semaglutide respectively) and their mechanisms differ in a way that matters clinically.

Semaglutide is a GLP-1 receptor agonist. It mimics a gut hormone that signals fullness, slows how quickly food leaves the stomach, and reduces appetite. Mounjaro does all of that too, but it also activates a second receptor: GIP. This dual action is why Mounjaro is described as a dual GIP and GLP-1 receptor agonist, and it is the only weight-loss injection in the UK that works on both pathways simultaneously. You can read a plain-English summary of how tirzepatide works on the NHS tirzepatide medicines page.

Why does the mechanism matter? Because the trial results differ. In the SURMOUNT-1 study, tirzepatide produced average body-weight reductions of around 20–21% over 72 weeks at the highest dose. In the STEP 1 study of semaglutide 2.4mg, the average reduction was around 15% over 68 weeks. Those numbers come from different populations and trial designs, so direct comparison needs care, but a head-to-head trial (SURMOUNT-5, published in the New England Journal of Medicine in 2025) did find greater average weight loss with tirzepatide. A shorter sentence: same category, meaningfully different performance on average.

Neither result is guaranteed for any individual. A prescriber weighs your health history, existing conditions, and how you respond to treatment, not just the headline percentage.

How the dosing schedules compare in practice

Both injections are taken once a week, subcutaneously, into the abdomen, thigh or upper arm, rotating sites each time. That similarity aside, the titration ladders are different.

Mounjaro comes in six strengths: 2.5, 5, 7.5, 10, 12.5 and 15mg. Treatment starts at 2.5mg, and most people step up every four weeks under their prescriber's guidance, working toward the dose that balances effectiveness with tolerability. The 2.5mg pen's job is to let your system adjust; the therapeutic work tends to build as the dose increases.

Wegovy's standard schedule runs from 0.25mg up through 0.5, 1.0 and 1.7mg to a 2.4mg maintenance dose. In January 2026 the MHRA approved a higher 7.2mg maintenance dose, initially delivered as multiple pens weekly, then via a dedicated single-dose 7.2mg pen approved in April 2026. That higher ceiling narrows the average results gap between the two injections, though 7.2mg is only licensed for adults with a BMI of 30 or above.

Storage is worth flagging practically: both injections need to be kept refrigerated between 2 and 8°C. A lot of people keep their pen in the fridge door. For exact guidance on how long each can safely sit at room temperature if needed, the Mounjaro or Wegovy Patient Information Leaflet is the right reference, those windows differ and the leaflet is always current.

For a broader look at how weight loss injections work in general, our overview covers the shared principles alongside the differences.

Which injection is licensed for whom, and what the prescriber is actually checking

Both injections share the same broad licensed criteria: adults with a BMI of 30 or above, or a BMI of 27–29.9 alongside at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. Those are the licence conditions, they describe the population each medicine has been studied and approved in.

What a prescriber actually checks goes further. Your full medical history, any medicines you already take, and conditions that may make one injection more or less suitable all feed into the assessment. There are contraindications (a personal or family history of medullary thyroid carcinoma or MEN2, for example, appears on both prescribing checklists) and some people's circumstances point more clearly toward one option than the other.

Women using oral contraceptives should know that tirzepatide (Mounjaro) may reduce pill absorption during the first four weeks of treatment and for four weeks after any dose increase; adding a barrier method during those windows is the guidance from NHS England. No equivalent evidence exists for semaglutide. This is the kind of detail that changes a prescribing conversation.

If you are considering the differences between weight loss injections from a clinical angle, that page goes deeper on the prescribing considerations. And if cost is part of your thinking, our page on injection pricing explains what legitimate private treatment actually includes.

The third option that often comes up, and two that shouldn't

Searches for the different kinds of weight loss injections often return results mentioning Saxenda (liraglutide) and Ozempic (also semaglutide). A quick factual note on each.

Saxenda is a GLP-1 injection licensed for weight management in the UK, it uses liraglutide and requires a daily injection rather than weekly. It predates both Mounjaro and Wegovy and is less commonly prescribed now that once-weekly options with stronger trial evidence are available. It is worth knowing it exists, but it is not dispensed by every clinic.

Ozempic also contains semaglutide, but it is licensed in the UK for type 2 diabetes management, not weight loss. The dose and pen format differ from Wegovy. Prescribing Ozempic off-label for weight loss is not appropriate, and if you want to understand the different types of weight loss injections that are legitimately available, obtaining anything from unverified sources online carries serious risks, as the MHRA has repeatedly warned about counterfeit GLP-1 pens circulating through unregistered sellers.

For a current rundown of what is actually available and prescribed, our page on current weight loss injections covers the licensed landscape as it stands. The treatment options page is where you can start a consultation if you want a prescriber to assess which is right for you.