Which is the best weight loss injection on the NHS — and what does the evidence actually say?

What the trial data says about how well each injection works

NICE's appraisal of tirzepatide drew heavily on the SURMOUNT-1 trial published in the New England Journal of Medicine, in which participants lost an average of around 20–21% of body weight over 72 weeks at the highest dose. That figure has since been tested head to head: the SURMOUNT-5 trial, also published in the NEJM in 2025, randomised 751 adults with obesity and no diabetes to either tirzepatide or semaglutide 2.4mg for 72 weeks. Tirzepatide produced the larger average weight reduction. No other comparison of the two medicines at these doses has been run in this way.

Semaglutide's STEP 1 trial, also 68 weeks long, showed an average reduction of around 15% of body weight at the 2.4mg weekly dose. Novo Nordisk has since received MHRA approval for a higher 7.2mg dose, and early data suggests roughly 20.7% average loss at that level, narrowing the gap with tirzepatide considerably. So the picture is still moving.

What does this mean practically? Tirzepatide holds the edge in head-to-head evidence right now. Whether that difference matters to an individual patient (and whether the right choice is tirzepatide at all) is a clinical conversation, not a headline.

NHS eligibility: the criteria are stricter than most people expect

The NHS does not offer both medicines on the same terms. Semaglutide (Wegovy) is available through specialist weight management services under NICE Technology Appraisal 875, for a maximum of two years, for adults with a BMI of 35 or above and at least one weight-related condition, when managed alongside a multidisciplinary team. Waiting lists for those services are long in most areas.

Tirzepatide is newer on the NHS. NICE TA1026, published in December 2024 and updated in September 2025, recommends it for adults with a BMI of 35 or above and at least one weight-related comorbidity. NHS England is rolling access out in phases: from June 2025, those with a BMI of 40 or above and four or more qualifying conditions (from a list including type 2 diabetes, high blood pressure, obstructive sleep apnoea, dyslipidaemia and cardiovascular disease) could access it; from around June 2026, that opened to BMI 35–39.9 with four or more conditions. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. Meeting the criteria does not guarantee a prescription, a clinician still assesses the whole picture, and availability varies by area.

You can read more about how NHS weight loss injections work in practice, including what wrap-around support is required.

What makes tirzepatide different from semaglutide clinically

The two medicines work through related but distinct pathways. Semaglutide activates GLP-1 receptors, which slow gastric emptying, reduce appetite and influence blood sugar. Tirzepatide activates both GLP-1 and GIP receptors, it is the only dual-agonist weight-loss medicine licensed in the UK. The addition of the GIP pathway appears to contribute to the larger weight-loss effect seen in trials, though the precise mechanism is still being studied.

Both are once-weekly injections administered under the skin, typically in the abdomen, thigh or upper arm. Both carry the Black Triangle designation from the MHRA, meaning they are under additional monitoring while longer-term safety data accumulates. If you are weighing up your options, it is worth reading about whether weight loss injections are considered safe by the NHS, including what the monitoring requirements mean for patients. Side effects in both cases are largely gastrointestinal, nausea, loose stools, constipation and indigestion are the most commonly reported, most pronounced in the early weeks of treatment or after a dose increase. The NHS tirzepatide patient page sets out what to watch for in plain language.

For a broader comparison of available weight loss injections in the UK, including how they differ on licensed indications and evidence, our treatment overview covers the full picture.

Private prescribing: when the NHS route isn't open to you

Most people asking this question don't meet the current NHS thresholds, or they meet them but face a wait measured in months. Private prescribing through a GPhC-registered pharmacy is a legal and regulated alternative. The medicine is the same; what changes is who prescribes it and who pays.

At nume (sorry, at a regulated private service like ours) the consultation is reviewed by a real prescriber, not automated. A few things worth knowing if you're considering this route: any legitimate provider requires a clinical assessment before issuing a prescription, will verify your identity and weight, and will supply from the licensed UK supply chain. If you are unsure where to start, our guide on where to buy weight loss injections safely explains what to look for in a reputable provider and what the red flags are. You can verify any online pharmacy on the GPhC register before you share any personal information.

It is also worth understanding whether you qualify for the NHS route before committing to private costs, the eligibility criteria have been expanding, and your GP may now be able to prescribe tirzepatide directly under the 2026 QOF contract changes. If you have questions about how private pricing compares across providers, that page covers what's typically included and what's often left out. When you're ready to explore your options, you can start a free consultation and a GPhC-registered prescriber at nume will review your answers the same day.