Which injection is good for weight loss in the UK?

You've searched this because something isn't working — here's the honest picture

You're probably here after hearing about weight-loss injections from a colleague, a news headline, or a GP who mentioned them in passing. Maybe you've spent time on waiting lists that haven't moved. Whatever brought you here, the question itself is the right one to ask: not which is cheapest, not which has the flashiest advert, but which one is actually good. If you want to get straight to that question, our guide to finding a good weight loss injection covers what separates genuinely effective options from the noise. That reframe matters.

Two injections are licensed for weight management by the MHRA in the UK. Mounjaro contains tirzepatide, manufactured by Eli Lilly. Wegovy contains semaglutide, manufactured by Novo Nordisk. Both slow gastric emptying, both reduce appetite, and both are taken as once-weekly injections using pre-filled pens. The NHS patient page on tirzepatide and its equivalent for semaglutide give a grounded starting point for what each medicine does.

Nothing else is licensed for this purpose as an injection in the UK. B12 injections, lipotropic shots, and various things sold informally online are not licensed weight-loss treatments. If you are curious about what the evidence says on that front, the B12 injections and weight loss page covers it plainly.

The mechanism gap: why one receptor or two changes the numbers

Wegovy works on the GLP-1 receptor, which is involved in insulin secretion, appetite signalling in the brain, and slowing digestion. That single-receptor action produced roughly 15% average weight loss over 68 weeks in the STEP 1 trial, a result that was considered landmark when it published. The STEP 1 paper in the New England Journal of Medicine remains the primary reference.

Mounjaro adds GIP receptor activation on top of that. GIP is a gut hormone with its own role in energy metabolism and fat storage. Whether the additive effect of two receptors is what drives the difference in outcomes is still being studied, but the results are measurable. In SURMOUNT-1, involving 2,539 adults with obesity, average body-weight reduction reached around 20–21% at the 15mg dose over 72 weeks. The head-to-head SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, confirmed that tirzepatide produced greater average weight loss than semaglutide 2.4mg directly compared over the same period.

Wegovy 7.2mg, approved in the UK in January 2026 and now available as a single-dose pen approved in April 2026, narrows that gap. It reported around 20.7% average loss over 72 weeks, approaching the tirzepatide 15mg figure. So the gap between the two medicines is tightening, which is worth knowing when you talk to a prescriber. A fuller breakdown is on the comparison page for weight-loss injections.

Eligibility: the BMI thresholds and conditions that determine who qualifies

Both injections share a similar licensed eligibility profile. Adults with a BMI of 30 or above qualify. Those with a BMI between 27 and 29.9 can also qualify if they have at least one weight-related condition, things like high blood pressure, type 2 diabetes, dyslipidaemia, or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance; NICE specifies a 2.5 kg/m² reduction for South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds.

BMI alone never decides the outcome. A prescriber looks at your medical history, any contraindications, existing medicines, and your overall health picture. That assessment is what separates a legitimate prescription from an online purchase that bypasses clinical oversight entirely. The MHRA has been clear that GLP-1 medicines are not assessed for cosmetic or quick weight loss, and the legal route requires a clinical evaluation.

If you want to think through what might suit your specific situation before a consultation, the guide to choosing the right weight-loss injection for you walks through the key factors. For a broader look at what private treatment typically involves cost-wise, the weight-loss treatments page sets that out without surprises.

Side effects, safety infrastructure, and what to watch for

Both injections share a broadly similar side-effect profile, dominated by gastrointestinal effects: nausea, loose stools, constipation, reflux, and occasionally vomiting. These tend to be most noticeable when starting or after a dose increase, and for most people they settle within days to a couple of weeks. Starting at a low dose and titrating slowly is how the prescribing schedule manages this.

More serious effects are less common but require attention. In January 2026 the MHRA issued a Drug Safety Update highlighting acute pancreatitis as a known, infrequent but potentially serious risk across GLP-1 medicines. Severe stomach pain that spreads to the back, with or without vomiting, needs urgent medical attention. Both injections also carry a black triangle (▼) designation in the UK, meaning they are subject to additional monitoring. Any suspected side effect can be reported at the MHRA Yellow Card scheme.

Neither medicine is suitable during pregnancy, while breastfeeding, or in the period before trying to conceive. Women on oral contraceptives starting tirzepatide should use an additional method for the first four weeks of treatment and for four weeks after each dose increase, as absorption of the pill may be affected. These are conversations to have with a prescriber, who can advise on your specific circumstances. Our page on whether weight-loss injections are right for you covers the wider safety picture in more depth, and our overview of what a weight loss injection actually involves is a useful starting point if you are still building your understanding.