Three injectable medicines have come to dominate patient questions in UK weight-management practice: Ozempic, Wegovy and Mounjaro. They are often talked about interchangeably online, but the differences matter — they are not the same molecule, they are not licensed for the same things, and the trial evidence behind them is not identical. This article walks through all three, side by side, in the way a UK clinician approaches the conversation.
The three injections at a glance
There are really only two active molecules in this conversation. Semaglutide is a GLP-1 receptor agonist sold under two different UK brands — Ozempic for type 2 diabetes and Wegovy for weight management. Tirzepatide is a dual agonist that acts on both the GLP-1 receptor and the GIP (glucose-dependent insulinotropic polypeptide) receptor. It is sold in the UK under the brand Mounjaro. So although the pharmacy shelf shows three named products, pharmacologically there are two ingredients — semaglutide (single-target) and tirzepatide (dual-target). For a fuller mechanism-level walk-through, our explainer on semaglutide vs tirzepatide covers the receptor biology in more depth.
| Feature | Ozempic | Wegovy | Mounjaro |
|---|---|---|---|
| Active ingredient | Semaglutide | Semaglutide | Tirzepatide |
| Mechanism | GLP-1 receptor agonist | GLP-1 receptor agonist | Dual GLP-1 + GIP receptor agonist |
| UK licensed use | Type 2 diabetes | Weight management | Type 2 diabetes and weight management |
| Injection frequency | Once weekly | Once weekly | Once weekly |
| Dose range | 0.25 mg to 1 mg (up to 2 mg) | 0.25 mg to 2.4 mg | 2.5 mg to 15 mg |
| Flagship weight trial | Not for weight indication | STEP-1: ~15% mean weight loss | SURMOUNT-1: ~22% mean weight loss |
| Common side effects | Nausea, constipation, diarrhoea | Nausea, constipation, diarrhoea | Nausea, constipation, diarrhoea |
| UK access route | NHS (T2D) or private | Specialist NHS pathway or private | Specialist NHS pathway or private |
What each is licensed for in the UK
Licensing is where the confusion often starts. In the UK, the MHRA sets out what a medicine can be prescribed for in its licensed indication. Ozempic is licensed for type 2 diabetes only. Wegovy is licensed for weight management in adults meeting specific BMI criteria — typically a BMI at or above 30, or at or above 27 with a weight-related comorbidity. Mounjaro is licensed for both type 2 diabetes and, more recently, for weight management with broadly similar BMI thresholds.
That distinction matters practically. When a UK clinician prescribes semaglutide for weight management, the licensed brand is Wegovy — not Ozempic. Using Ozempic for weight management is an off-licence decision that has to be justified, documented and monitored. It also has knock-on effects: Ozempic supply is intended for people with type 2 diabetes, and unlicensed use can affect availability for the group the medicine is licensed for. NICE guidance and the BNF reflect the same licensed framework, and a responsible pharmacy prescribes within it.
Trial-level weight loss compared
The most cited numbers come from the flagship weight-management trials. Semaglutide 2.4 mg was studied in the STEP-1 trial, which reported mean weight loss of approximately 15 percent from baseline over 68 weeks with once-weekly injection plus lifestyle support. Tirzepatide was studied in SURMOUNT-1, which reported mean weight loss of approximately 22 percent at the 15 mg once-weekly dose over 72 weeks, again with lifestyle support. Ozempic does not have a headline weight-management trial of its own, because it is not licensed for that indication.
Two caveats matter. First, STEP-1 and SURMOUNT-1 were separate trials in different populations, not a direct head-to-head. More recent trials have compared the two more directly and broadly support the direction of that signal. Second, these are trial averages — individual response is highly variable. Some people on semaglutide lose more than the average tirzepatide response; some people on tirzepatide plateau earlier than the average semaglutide response. A number is not a promise. Our article on how long Mounjaro takes to work gives a sense of realistic timescales.
Dosing schedule and titration
All three follow a slow, structured titration — starting at a low dose and stepping up every four weeks — with the aim of reaching a maintenance dose after several months. That schedule is not decoration. It is the reason side effects stay manageable for most people.
Wegovy titrates from 0.25 mg through 0.5 mg, 1 mg and 1.7 mg to the maintenance dose of 2.4 mg once weekly, over roughly 16-20 weeks. Ozempic titrates from 0.25 mg through 0.5 mg and 1 mg, with the option of 2 mg for type 2 diabetes if glucose control needs it. Mounjaro titrates from 2.5 mg through 5 mg, 7.5 mg, 10 mg and 12.5 mg to the top dose of 15 mg once weekly. Not everyone reaches the top dose — some people stabilise earlier and stay there. Our companion piece on when to increase a Mounjaro dose covers the decision points around titration.
Skipping steps or self-adjusting is not appropriate. If a dose increase is not well tolerated, the usual approach is to hold the current dose for another cycle before stepping up, not to abandon the medicine.
Side effects — where they overlap and differ
The three share most of their common side effects because both semaglutide and tirzepatide slow gastric emptying and act on GLP-1 signalling. Nausea, constipation, diarrhoea, reflux and occasional vomiting are the most commonly reported. They tend to be worst in the days after a dose step-up and settle over the following weeks. Fatigue and a reduced appetite are expected effects rather than problems — the medicine is doing what it is designed to do.
Less common but recognised concerns for all three include acute pancreatitis, gallbladder events (particularly with larger amounts of weight loss), and significant dehydration if vomiting or diarrhoea persist. All three carry precautions around medullary thyroid cancer and multiple endocrine neoplasia type 2 based on animal data. The BNF and each product's summary of product characteristics set out the full list, and any concerning symptom during treatment warrants clinical review.
Cost and access in the UK
NHS access for weight management is limited and pathway-dependent. Wegovy is available through specialist tier-3 weight-management services in England for eligible patients, with strict BMI and comorbidity criteria and time-limited use. Mounjaro's NHS availability for weight management is being rolled out through specialist pathways with similar eligibility rules. Ozempic on the NHS is prescribed for type 2 diabetes only.
Through a UK-registered online pharmacy such as Farmeci, all three are private prescription medicines. Pricing is transparent per pen and varies by strength. A clinician-led consultation is required before supply, and the pharmacy is expected to prescribe within the licensed indication, screen for eligibility, and provide follow-up review. Cost sits alongside — not above — clinical fit in the decision.
How UK clinicians match a patient to a molecule
Choosing between Ozempic, Wegovy and Mounjaro is a clinical decision, not a shopping choice. A UK clinician will weigh the licensed indication that fits your case first — if the primary goal is weight management, that narrows the field to Wegovy or Mounjaro. Your medical history, BMI, prior weight-management treatments and how you tolerated them, comorbidities (particularly cardiovascular, gastrointestinal and renal), other medicines you take, monitoring capacity, supply and cost all feed into the decision.
Higher average trial weight loss is one data point among several. Tolerability at earlier doses often matters more than headline numbers at the top dose, because a medicine you cannot take at the top dose does not deliver its top-dose benefit. If you have already tried one and it did not suit you, that history is highly relevant to the choice of the next step. Your clinician will advise based on your individual circumstances, and for some people the appropriate answer is that none of these three is the right treatment right now. Our article on staying on a low dose of Wegovy illustrates how these decisions play out at the maintenance stage.
Beyond weight — cardiometabolic context
All three medicines have shown broader cardiometabolic effects — improvements in glucose control, blood pressure, lipid profile and inflammatory markers in the relevant patient groups. For people with type 2 diabetes, Ozempic and Mounjaro are also licensed for glucose control, and the prescribing conversation is different again. A weight-management consultation is not a diabetes consultation, and one is not a substitute for the other, but the overlap is part of why these medicines have become as clinically prominent as they have.