Which Is the Best Weight Loss Injection in the UK Right Now?

What the trial evidence actually says about which injection produces more weight loss

This is the question most people want answered first, so here it is plainly. In the SURMOUNT-1 trial, adults taking tirzepatide (Mounjaro) at the highest dose lost an average of around 20–21% of their body weight over 72 weeks. In the STEP 1 trial for semaglutide (Wegovy) at 2.4mg, average weight loss was around 15% over 68 weeks. Both trials involved adults with obesity but without type 2 diabetes, and both used lifestyle support alongside the medicine.

The most direct comparison comes from the SURMOUNT-5 trial, published in the New England Journal of Medicine (2025), which put tirzepatide and semaglutide 2.4mg head-to-head in over 750 adults. Tirzepatide produced greater average weight reduction. That result informed NICE's appraisal of tirzepatide, and it is the reason most clinicians would consider Mounjaro the stronger option on the evidence base alone.

The caveat matters, though. Averages are not predictions. Some people respond better to semaglutide; some do not tolerate tirzepatide's dose-escalation well. A single trial average is a starting point for a clinical conversation, not a prescription. If you want a fuller breakdown of which factors tend to separate these two medicines in practice, the evidence comparison across both injections goes into more detail.

The biological reason Mounjaro and Wegovy work differently

Wegovy (semaglutide) is a GLP-1 receptor agonist. It mimics a gut hormone that signals fullness, slows how quickly food leaves your stomach, and affects appetite centres in the brain. It has a strong track record, and GLP-1 medicines as a class have been studied in large trials for over a decade.

Mounjaro (tirzepatide) does all of that and also activates the GIP receptor, a second gut-hormone pathway involved in energy balance and fat storage. It is the only dual-agonist weight-loss injection licensed in the UK. Whether that second mechanism is why Mounjaro tends to outperform Wegovy on average weight loss is still being studied, but the difference in mechanism is real and established. The NHS medicines page for tirzepatide explains this clearly at a patient level if you want a straightforward reference.

In practical terms, both injections are given once a week, subcutaneously (under the skin), using a pre-filled pen. Both start at a low dose and increase gradually under clinical guidance to help your body adjust. The question of where to inject is the same for both: abdomen, thigh or upper arm, rotated each week. Neither requires daily administration, which is one reason many people find a weekly injection more manageable than daily tablets.

Eligibility: who can be prescribed which injection

Both medicines share a similar licensed eligibility frame for UK private prescribing: adults with a BMI of 30 or above, or adults with a BMI of 27 or above alongside at least one weight-related health condition, such as high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance.

On the NHS the picture is different. NICE's appraisal of tirzepatide (TA1026) recommends it for adults with a BMI of at least 35 plus a weight-related comorbidity, and the NHS is rolling out access in phases. Wegovy sits under a separate NICE recommendation (TA875), available through specialist weight management services with a two-year maximum duration. Waiting times for NHS access are commonly long, and many people choose private treatment rather than wait. You can read more about how weight loss injections work and who they suit if you are still weighing up your options.

One practical note: if you are thinking about starting around a particular date, payday or a holiday, ordering before the 12pm weekday cut-off means treatment can be dispatched the same day once a prescriber has reviewed your consultation. That timing detail is worth knowing before you plan.

For people who want help thinking through which injection fits their personal circumstances (not just the headline trial numbers) the individual factors that influence which injection suits you covers BMI, health history, tolerance and more.

Cost, access and what to check before choosing a provider

Price matters, and it is worth being honest about it. Since Eli Lilly raised Mounjaro's UK list price in September 2025, reported private market prices range roughly £149–£375 per month depending on dose and provider. Wegovy sits in a comparable range. Neither is cheap, and anyone offering these medicines far below market rates warrants serious scrutiny.

These are prescription-only medicines. A provider charging unusually little may be cutting corners on clinical oversight, using an unregulated supply chain, or both. The MHRA has seized approximately 20 million doses (around £45 million street value) of illegally traded weight-loss medicines in the UK. Fake pens have contained insulin. The right question when comparing providers is not which is cheapest but which is running a proper clinical process and sourcing through the licensed UK supply chain.

You can verify any online pharmacy on the GPhC register, our pharmacy registration number is 9012878 if you want to check us. If you are comparing providers rather than just medicines, the factors that separate legitimate providers from risky ones is worth reading before you commit.

At nume, every consultation is read by a real GPhC-registered prescriber the same day, not processed by software. If you are clinically suitable and approve before midday on a weekday, your treatment is dispatched that day and arrives free by tracked DPD the next working day. The single price includes consultation, prescription, delivery and seven-day aftercare support, with no subscription and no auto-renewal. Check your eligibility and see what that process looks like in practice.