If you have followed the weight-loss injection story at all, you will have heard three names over and over: Ozempic, Wegovy and Mounjaro. They are often used almost interchangeably, as if they were rival versions of the same thing. In reality the picture is more interesting — two of them are the same drug wearing different labels, one is a genuinely different medicine, and one of the three is not licensed for weight loss in the UK at all. Here is a clear, impartial guide to what actually separates them.
The short version
All three are once-weekly injections from the same broad family — GLP-1 receptor agonists — which work by mimicking a natural gut hormone that curbs appetite, slows how quickly the stomach empties and helps regulate blood sugar. Where they differ is the exact drug inside, the dose, and crucially what each is licensed to be used for.
| Ozempic | Wegovy | Mounjaro | |
|---|---|---|---|
| Active drug | Semaglutide | Semaglutide | Tirzepatide |
| Drug class | GLP-1 agonist | GLP-1 agonist | Dual GIP/GLP-1 agonist |
| Manufacturer | Novo Nordisk | Novo Nordisk | Eli Lilly |
| UK licence | Type 2 diabetes only | Weight management | Weight management |
| Injection frequency | Once weekly | Once weekly | Once weekly |
| Average weight loss in trials | ~5–7% (off-label) | ~15% | ~20–22% |
| Licensed for weight loss in UK? | No | Yes | Yes |
The single most useful thing to understand is that Ozempic and Wegovy are the same drug, semaglutide. Mounjaro is a different, newer molecule. Once that clicks, the rest falls into place.
Ozempic vs Wegovy: one drug, two jobs
Both Ozempic and Wegovy are semaglutide, made by Novo Nordisk. The difference is the dose and the licence.
Ozempic is licensed in the UK exclusively for type 2 diabetes, at doses up to 2 mg a week. It was never approved here as a weight-loss treatment. Because semaglutide reduces appetite as a side effect of managing blood sugar, people taking it for diabetes often lose weight too — which is how Ozempic became famous. But that fame created a problem: demand for a diabetes medicine as a slimming aid contributed to shortages, making it harder for some people with diabetes to get the treatment they rely on.
Wegovy is the same molecule reformulated and licensed specifically for weight management, at a higher maintenance dose of 2.4 mg a week (a 7.2 mg version was approved by the MHRA in early 2026). Because it can be escalated to that higher weight-management dose, Wegovy produces more weight loss than Ozempic does at its diabetes doses. In its pivotal STEP trials, semaglutide 2.4 mg produced average weight loss of around 15%, and it is the version the NHS and reputable private clinics use when the goal is weight loss.
Because Ozempic and Wegovy contain the same drug, using Ozempic “for weight loss” is essentially an off-label workaround — one that bypasses the proper weight-management dose and support, and diverts a diabetes medicine from the people who need it.
There is one further practical difference worth knowing: Ozempic typically comes as a reusable multi-dose pen, while Wegovy is usually a single-use disposable pen. Same active ingredient, different device and, more importantly, different legal purpose.
Mounjaro: the dual-action newcomer
Mounjaro is where the picture genuinely changes. Its active drug is tirzepatide, made by Eli Lilly, and it is not semaglutide at all. What makes it different is that it acts on two gut-hormone receptors rather than one: alongside GLP-1, it also activates the GIP receptor. This dual action appears to work synergistically, which is why tirzepatide has consistently outperformed pure GLP-1 drugs in head-to-head research.
The numbers are striking. In the SURMOUNT-1 trial, participants on the highest dose of tirzepatide lost an average of 22.5% of their body weight over 72 weeks, with the 10 mg dose close behind at around 21%. That is meaningfully more than the roughly 15% seen with Wegovy, and it is why tirzepatide is often described as the most effective weight-loss medication approved so far.
Mounjaro was authorised by the MHRA for weight management in November 2023, recommended by NICE (guidance TA1026) in December 2024, and — unlike Wegovy — has begun a phased rollout into NHS primary care, meaning some GPs can now prescribe it for eligible patients. It is a once-weekly injection, started low at 2.5 mg and increased gradually every few weeks up to a maximum of 15 mg.
A quick word on Saxenda
You may also come across Saxenda (liraglutide). It was the first GLP-1 weight-management injection licensed in the UK, back in 2017, and it still has a role — but it is a daily injection rather than weekly, and produces less average weight loss (around 8%). It has largely been overtaken by the newer once-weekly options, though it may suit people who cannot tolerate them.
How they compare on weight loss
Ranking the three named medicines by average weight loss in clinical trials is straightforward, but it is only part of the story:
- Mounjaro (tirzepatide): roughly 20–22% average — the highest of the group.
- Wegovy (semaglutide 2.4 mg): roughly 15% average.
- Ozempic (semaglutide, diabetes doses): roughly 5–7% off-label, because it cannot be escalated to the weight-management dose.
Two important caveats sit underneath those figures. First, trial averages are not guarantees — individual results vary widely, and a proportion of people respond only modestly. Second, weight loss in these trials was always achieved alongside a reduced-calorie diet and increased activity, not instead of them. The medicine makes those changes easier to sustain; it does not replace them.
Side effects and safety — what they share
Because they belong to the same class, all three share a broadly similar safety profile. The most common effects are gastrointestinal: nausea, vomiting, diarrhoea and constipation, usually most noticeable while the dose is being increased and often settling as the body adjusts. Roughly three in four people experience no nausea or only mild, manageable symptoms.
Rarer but more serious risks apply across the whole class and should be discussed with a prescriber before starting. These include pancreatitis, an increased risk of gallstones (partly linked to rapid weight loss), and a precautionary warning around thyroid tumours that makes these medicines unsuitable for anyone with a personal or family history of medullary thyroid cancer or the MEN2 syndrome. They are also not suitable in pregnancy or breastfeeding.
There are cosmetic knock-on effects too, sometimes nicknamed “Ozempic face” — facial volume loss and loose skin can follow rapid weight reduction, and some muscle is lost alongside fat, which is why resistance exercise and adequate protein are usually recommended. None of this is a reason to avoid treatment, but it is a reason to go in with clear expectations.
Who they’re for — and the counterfeit risk
These are prescription-only medicines intended for people living with obesity, or with a raised BMI plus a weight-related health condition, and they are meant to be used under proper medical supervision — not casually, and not to lose a few vanity pounds. A responsible prescriber will assess your BMI, medical history, current medications and suitability before considering any of them, and will review your progress over time.
That supervision matters more than ever given the surge in demand. The MHRA has repeatedly warned about counterfeit weight-loss pens sold through social media and unregulated websites. Fakes have been found to contain the wrong drug, an unknown dose, or no active ingredient at all, and some have caused serious harm. If a deal looks cheap and requires no proper consultation, treat it as a red flag. Genuine treatment always comes through a registered pharmacy or clinic with a real prescriber behind it.
It is also worth remembering the honest limitation that applies to all of these medicines: the evidence shows weight is commonly regained after stopping, because they work while they are being taken. They are a long-term tool, not a short course — which makes the quality and continuity of your clinical care central to whether they help you in the long run.
Making an informed choice
So, the difference in a sentence: Ozempic and Wegovy are the same drug (semaglutide) licensed for different jobs, while Mounjaro is a different, dual-action drug (tirzepatide) that delivers the highest average weight loss — and of the three, only Wegovy and Mounjaro are licensed for weight loss in the UK. Which, if any, is appropriate is a medical decision, not a shopping one, and it belongs with a suitably qualified prescriber who can weigh up your individual health.
If you are exploring this route, it is worth reading our companion guides on what Mounjaro (tirzepatide) is, whether weight-loss injections are safe, and how these medicines compare with weight-loss surgery for a fuller picture. Above all, choose your provider carefully. Use Clinic Finder to compare properly regulated clinics near you and check your prescriber’s credentials before you commit to anything — because with prescription weight-loss medicines, who supervises your care matters just as much as which one you take.
