Are weight loss injections safe in the UK?

You've read the headlines and you're not sure what to believe

Perhaps a friend mentioned they'd started one of the new injections and you felt a mix of curiosity and caution. Or you've seen the stories about side effects and wonder whether the risks outweigh the benefits for someone in your situation. That uncertainty is completely reasonable, these are real medicines, not supplements, and treating them seriously is exactly the right instinct.

Mounjaro (tirzepatide) and Wegovy (semaglutide) were each granted UK marketing authorisations by the MHRA following phase-three clinical trial programmes. SURMOUNT-1, which studied tirzepatide in adults with obesity over 72 weeks, enrolled more than 2,500 participants; the STEP 1 trial for semaglutide was similarly large. Both were powered to detect not just whether the medicines worked, but whether they were well-tolerated and what their side-effect profile looked like. The NHS medicines pages for both tirzepatide and semaglutide set out that profile clearly, and they make useful reading alongside any consultation.

The safety picture that emerges is consistent: most side effects are gastrointestinal, most are mild to moderate, and most ease as the body adjusts. Serious effects are less common but real. That balance is what the MHRA weighed when granting licences, and it is what a prescriber weighs when deciding whether treatment is appropriate for a specific individual. If you are weighing up whether weight loss injections are safe for someone in your circumstances, that question is best answered through a proper clinical assessment rather than a single yes or no.

Our guide to weight loss injections covers both medicines in more detail if you want to understand the differences before a consultation.

What side effects are genuinely common, and which ones need prompt attention

Knowing what to expect makes side effects easier to manage and helps you spot anything that needs medical attention sooner. The side effects that occur most often with both tirzepatide and semaglutide share a clear pattern: nausea, vomiting, diarrhoea, constipation, indigestion, burping, and fatigue. Headaches and dizziness appear too, particularly in the first weeks. Injection-site reactions (mild redness or discomfort) are worth mentioning but rarely a reason to stop.

These effects are most likely to surface when you start treatment or when your dose goes up, because the medicine slows gastric emptying and your digestive system is adapting. For many people they settle within a week or two. Eating smaller portions, avoiding very fatty meals, and staying well hydrated all help.

Then there are the effects that are less frequent but serious. In January 2026 the MHRA published a Drug Safety Update specifically flagging acute pancreatitis as a known risk with GLP-1 medicines, infrequent, but worth understanding. The warning sign is severe stomach pain that spreads to your back, with or without vomiting. That warrants a call to 111 or a visit to A&E, not a wait-and-see approach. Gallbladder problems, signs of a serious allergic reaction (swelling of the face or throat, difficulty breathing), and significant dehydration from persistent vomiting or diarrhoea are in the same category: seek medical help quickly. You can read more about what to watch for on our page on the newer injections' safety profile.

Anyone who experiences side effects they are unsure about can report them directly to the MHRA via the Yellow Card scheme. That reporting feeds back into the ongoing monitoring of these medicines and benefits future patients.

Who these medicines are licensed for, and why that boundary matters

Licensing criteria exist for a reason. Both Mounjaro and Wegovy are licensed for adults with a BMI of 30 or above, or from 27 with at least one weight-related health condition such as high blood pressure, high cholesterol, or type 2 diabetes. Lower BMI thresholds can apply for some ethnic backgrounds under UK guidance. People who are pregnant, breastfeeding, or trying to conceive should not use these medicines. They are not licensed for anyone under 18. Certain conditions (a history of pancreatitis, medullary thyroid carcinoma, or MEN2 syndrome) are among those that require careful prescriber assessment or rule treatment out entirely.

These boundaries are not bureaucratic hurdles. They reflect what the trial evidence showed about benefit versus risk for specific populations. A prescriber who assesses you properly before prescribing, and reviews your progress before each repeat, is the mechanism that keeps those boundaries real rather than theoretical.

If you are curious about whether the injections are suitable for your situation, our overview of the weight loss injection covers eligibility in plain terms, and questions about individual suitability are exactly what the free consultation at our prescribers is there to answer.

The counterfeit risk: why supply source is part of the safety question

One thing that does not always make it into discussions about injection safety is where the pen comes from. The MHRA has seized approximately 20 million doses of illegally traded weight loss medicines (with a street value of around £45 million) and, in October 2025, found the first UK site manufacturing counterfeit tirzepatide pens, in Northampton. Fake pens have been found to contain insulin, among other undisclosed substances. Several people needed hospital treatment.

Buying from a seller who does not require a valid prescription is not a cheaper version of the same thing. It is a categorically different and more dangerous thing. The safest route to these medicines in the UK is through a pharmacy you can verify on the GPhC register, with a prescription issued by a real clinician who has assessed your health. If you want to check how to find and verify a legitimate online pharmacy, our page on where to buy weight loss injections walks through what to look for.

At nume, every consultation is reviewed by a GPhC-registered Independent Prescriber (a real clinician reads your answers, not software) and all medicines come through the licensed UK supply chain. If you'd like to talk through your situation and whether treatment is right for you, the free consultation is the place to start. Our clinical team discusses suitability and side effects as part of every review, and aftercare is available seven days a week if questions come up later. You can also read more about the safety of using weight loss injections before you decide.