What are the main weight loss injections available in the UK?

You've seen the headlines, here's what the two injections actually are

Picture this: you've typed 'weight loss injection' into a search engine and come back with half a dozen names, some of them the same drug with different branding, some of them not licensed in the UK at all. It's confusing. So let's be precise.

The two injectable medicines licensed by the MHRA specifically for weight management in UK adults are Mounjaro (the brand name for tirzepatide, made by Eli Lilly) and Wegovy (the brand name for semaglutide at weight-management doses, made by Novo Nordisk). You may also hear about Ozempic, that's also semaglutide, but licensed for type 2 diabetes, not weight loss. If you want a clear explanation of what the weight loss injections are, the NHS medicines pages for tirzepatide and semaglutide both set this out, and it's a distinction worth holding onto before you read anything further.

Mounjaro comes as a pre-filled KwikPen in six strengths: 2.5, 5, 7.5, 10, 12.5 and 15 mg. Treatment starts at the lowest dose to let the body adjust, then the prescriber titrates upward in four-week steps. Wegovy follows a similar step-up approach, from 0.25 mg through to a 2.4 mg maintenance dose, and since April 2026 there is also a 7.2 mg single-dose pen approved by the MHRA for adults with a BMI of 30 or above. A look at the newer injection options covers the 7.2 mg development in more detail if that's what brought you here.

Both are Black Triangle medicines, meaning the MHRA collects additional safety data on them. That's normal for recently approved treatments and not a reason for alarm, it means more scrutiny, not more danger.

The mechanism behind the appetite change

A question our prescribers hear most weeks is some version of: 'but how does an injection stop me feeling hungry?' The short answer is that it doesn't simply suppress appetite by force. Both medicines mimic hormones your gut already produces after eating.

Wegovy works on one of those hormones: GLP-1 (glucagon-like peptide-1). When GLP-1 receptors in the brain and gut are activated, appetite falls, feelings of fullness arrive sooner, and gastric emptying slows, meaning food takes longer to leave the stomach. Mounjaro does all of that, and also activates GIP (glucose-dependent insulinotropic polypeptide) receptors. GIP has its own role in energy metabolism and fat storage, and activating both pathways together appears to produce a stronger effect on body weight than activating GLP-1 alone. Mounjaro is the only dual-agonist weight-loss medicine licensed in the UK.

The trial evidence reflects this. In the SURMOUNT-1 trial (2,539 adults with obesity over 72 weeks), participants on the 15 mg dose lost an average of around 20–21% of their body weight. The STEP 1 trial for semaglutide 2.4 mg (Wegovy) reported an average reduction of around 15% over 68 weeks. In the more recent SURMOUNT-5 head-to-head trial, published in the New England Journal of Medicine in 2025, tirzepatide produced greater average weight loss than semaglutide 2.4 mg directly. These are trial averages, individual results vary, and no medicine guarantees a specific outcome.

If you're comparing the two in more depth, this page on the two main weight loss injections sets the clinical differences side by side.

Who these injections are licensed for

Eligibility is a clinical question, not just a number. But the licensed criteria give a starting point. Both Mounjaro and Wegovy are licensed for adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition, for example, high blood pressure, type 2 diabetes, high cholesterol, or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance.

NICE has assessed both medicines and made recommendations about NHS access, but the NHS criteria are narrower than the licensed indications and are being phased in gradually. Many people who are clinically suitable under the licence don't yet meet the NHS thresholds. The broader guide to weight loss injections covers the NHS versus private routes if you want the full picture on access. For a practical overview of what's involved in starting treatment through a regulated private service, our weight loss treatment overview is a good next step.

For context on what private treatment typically costs, this page on the cost of weight loss injections explains what prices usually include and what to watch for. The point worth making here: both medicines are prescription-only, so any seller offering them without a clinical assessment first is operating outside the law. The MHRA has warned repeatedly about fake and illegally sold weight-loss pens, and a Yellow Card report is the right channel if you encounter one.

Side effects both injections share, and what differs

The most common side effects of both Mounjaro and Wegovy are gastrointestinal: nausea, constipation, diarrhoea, indigestion, burping and, for some people, vomiting. These tend to be most noticeable after starting treatment or after a dose increase, and they often ease within a week or two as the body adjusts. Fatigue, headache and dizziness are also reported.

More serious but less common effects include gallbladder problems and, rarely, acute pancreatitis. The MHRA issued a Drug Safety Update in January 2026 specifically flagging pancreatitis: severe stomach pain that radiates to the back, with or without vomiting, warrants urgent medical attention. The full overview of all weight loss injections covers the safety profile in more detail, including the contraception note for tirzepatide.

Women using oral contraceptives who start Mounjaro are advised to add a barrier method for the first four weeks of treatment and for four weeks after each dose increase. This is because tirzepatide slows gastric emptying enough to affect pill absorption. NHS guidance notes there is no equivalent evidence for this with semaglutide, but both medicines carry guidance to avoid use in pregnancy. Breastfeeding and trying to conceive are also situations where these medicines are not recommended. If any of those circumstances apply, that conversation belongs with your prescriber before starting anything.

Both medicines require safe disposal of used needles. A 1-litre sharps container is the standard way to handle that at home, in line with NHS guidance on safe sharps disposal.