What injection is used for weight loss in the UK?

The myth that any 'slimming jab' will do the same job

Ask around and you'll hear people talk about weight-loss injections as though they're a single category of drug, interchangeable, equally strong, varying only in brand name. That's not accurate. Tirzepatide and semaglutide belong to overlapping but distinct drug classes, and the practical differences are clinically relevant.

Semaglutide is a GLP-1 receptor agonist. It mimics a gut hormone released after eating, slowing gastric emptying and signalling fullness to the brain. It's been studied extensively, with the STEP 1 trial in the New England Journal of Medicine reporting an average body-weight reduction of around 15% over 68 weeks at the 2.4 mg maintenance dose.

Tirzepatide adds a second pathway. As a dual GIP and GLP-1 receptor agonist, it engages two gut-hormone receptors involved in appetite regulation and blood-sugar control. In the SURMOUNT-1 trial, average weight reductions of around 20–21% were recorded at the 15 mg dose over 72 weeks, figures that led NICE to recommend it in December 2024. A question our prescribers hear most weeks is whether tirzepatide is simply "stronger" than semaglutide. The honest answer is that it produced greater average weight loss in a head-to-head trial (SURMOUNT-5, 2025), but the right medicine for any individual depends on their full clinical picture.

Both are once-weekly injections administered under the skin (abdomen, thigh, or upper arm) and both start at a low tolerability dose before the prescriber titrates upwards.

Which bodies decided these injections could be used for weight loss in the UK?

Licensing in the UK sits with the MHRA. If you want a clear overview of the specific injections used for weight loss, understanding which products hold a genuine UK marketing authorisation for that purpose is the right place to start. That distinction is important because other injectable medicines that turn up in online searches, such as Ozempic, are not licensed for weight loss in the UK; Ozempic contains semaglutide but its UK licence covers type 2 diabetes. If you want to understand the overlap between diabetes injections and weight-loss treatment, the licensing differences are the right starting point.

Beyond the licence, NICE has reviewed both medicines for NHS use. Tirzepatide received a positive appraisal under NICE technology appraisal TA1026, published December 2024, recommending it for adults with a BMI of 35 or above plus at least one weight-related condition. Semaglutide was appraised earlier under TA875, with a recommendation that includes a two-year maximum duration and a requirement for specialist weight management services. NHS access remains phased and criteria are strict; private routes through a regulated pharmacy are an option for people who don't meet NHS thresholds or prefer not to wait.

Private eligibility, per the medicines' SmPCs, is explained fully on our page covering what weight is needed to qualify for weight-loss injections, including the lower BMI thresholds that apply for some ethnic backgrounds under UK guidance. A prescriber makes the final call.

Side effects and what to watch for

Both injections share a broadly similar side-effect profile, dominated by gastrointestinal symptoms: nausea, loose stools, constipation, reflux, and burping are common, particularly after starting or after a dose increase. These tend to settle within a week or two for most people. Fatigue, headache and injection-site reactions are also reported.

Less common but serious: the MHRA has flagged acute pancreatitis as a known risk with GLP-1 medicines, severe or persistent stomach pain that radiates to the back, with or without vomiting, warrants urgent medical attention. Suspected side effects can be reported via the MHRA Yellow Card scheme, which also accepts reports of suspect sellers. The NHS medicines page for tirzepatide and the patient information leaflet supplied with your medicine are the authoritative references for the full side-effect list; a prescriber will go through what to expect before you start.

People who are pregnant, breastfeeding, or planning a pregnancy are not recommended to use these injections. Neither is licensed for under-18s. Women taking oral contraceptives should add a non-oral method for the first four weeks of tirzepatide treatment and for four weeks after each dose increase, because tirzepatide may reduce absorption of the pill. NHS guidance does not identify the same concern for semaglutide, but your prescriber can advise based on your specific situation.

If you're weighing up which injection might suit your circumstances, our guide on choosing between the available options covers the clinical factors worth considering before your consultation. For a fuller picture of what private treatment costs in the UK and what an honest price includes, that page also addresses the things headline figures often leave out.

How to access a licensed weight-loss injection safely

The only lawful route to either of these injections in the UK is a prescription, issued following a proper clinical assessment. That can happen through an NHS pathway (subject to eligibility and waiting times) or through a private prescriber at a GPhC-registered pharmacy.

When going private, verifying the pharmacy on the GPhC register before handing over any personal or health information is a straightforward safety step. Sellers offering these medicines without requiring a prescription, via social media, or at prices far below the market are red flags. The MHRA has seized approximately 20 million doses of illegally traded weight-loss medicines with a street value around £45 million; fake pens have in some cases contained insulin, causing hospitalisations. The risk is real.

At nume, every consultation is personally reviewed by a GPhC-registered Independent Prescriber, a real clinician reads your answers, the same day. If approved, treatment is dispatched by 5pm and delivered free the next working day by DPD, in plain packaging. Our clinical team is built around that standard of oversight, not around speed for its own sake. To find out whether you're eligible, you can start your free consultation with no obligation.