What's the most effective weight loss injection available in the UK?

You've searched the comparison, here's what the trials actually found

Most people searching this question have already read conflicting claims online. Some sites quote percentage weight loss without naming the trial, the dose, or how long participants followed the treatment. So let's be concrete.

The SURMOUNT-1 trial followed 2,539 adults with obesity over 72 weeks. At the 15mg dose of tirzepatide, average body weight fell by around 20–21%. The STEP 1 trial for semaglutide 2.4mg, also 72 weeks, reported average losses of roughly 15%. Those figures come from the SURMOUNT-1 paper in the New England Journal of Medicine and the equivalent STEP 1 publication respectively.

The SURMOUNT-5 trial then compared the two medicines directly in adults with obesity but without type 2 diabetes, over 72 weeks, and tirzepatide again produced the greater average reduction. That gap is real, but it comes with an important caveat. These are averages. Individual response varies considerably, and some people do better on semaglutide than others do on tirzepatide's highest dose.

A newer option worth knowing about: the MHRA approved a 7.2mg Wegovy pen in April 2026, with trial data reporting average losses of around 20.7% at that dose, narrowing the distance between the two medicines. If you want to understand how effective weight loss injections have been shown to be across the clinical evidence, that's a useful place to get a grounded sense of the numbers before making any decisions. You can also explore how both injections work in practice for a fuller picture of what treatment actually involves week to week.

Why tirzepatide's dual mechanism matters, and when it might not

Tirzepatide is described as a dual GIP and GLP-1 receptor agonist. In plain terms, it activates two separate gut-hormone pathways that regulate appetite and blood-sugar response, whereas semaglutide activates one. The NHS explains this distinction clearly in its patient information on tirzepatide. That dual action is the likely reason for the difference in average trial outcomes.

But mechanism isn't destiny. Tolerability matters just as much as efficacy on paper. Both medicines share a broadly similar side-effect profile, nausea, constipation, and other gastrointestinal symptoms are the most commonly reported, particularly in the early weeks or after a dose increase. For some people those symptoms are brief and manageable; for others they lead to pausing or stepping back on the dose. If you're weighing up which injection is considered the most effective weight loss injection for your situation, tolerability is just as important a factor as headline trial figures.

Semaglutide also has a longer safety and prescribing record in the UK. Wegovy received its UK licence before Mounjaro, so there is slightly more real-world data in circulation. That doesn't make it superior, but it's a factor some clinicians and patients weigh when they're not sure which to try first.

For a direct, side-by-side look at how the two medicines are licensed and dosed, comparing the two by licence and dose structure sets out the specifics without the marketing noise.

What 'effective' actually means once treatment starts

Effectiveness in a trial is one number. Effectiveness in your life is a different question entirely.

Both injections are taken once weekly as a subcutaneous injection, starting at a low tolerability dose and increasing over several months. Your prescriber decides when (and whether) to move to the next level based on how you're responding and how you're feeling. Titration isn't automatic; it's a clinical judgement. Treatment sits alongside a reduced-calorie diet and more physical activity; the injection reduces appetite substantially, but what you eat alongside it shapes the result.

Neither medicine is licensed for quick cosmetic weight loss. The MHRA is explicit on this point. And neither is suitable for everyone. Pregnancy, breastfeeding, a history of certain thyroid conditions or pancreatitis, and some other medical factors mean one or both medicines may not be appropriate. An honest prescriber will tell you if they don't think the evidence supports treatment for you, and that conversation is what a legitimate consultation is for. If you're wondering about the broader range of weight loss treatments available through a clinician-led service, that's the right place to start.

On cost: private treatment prices vary considerably by provider and dose. For context on what the UK market looks like and what a legitimate prescription service actually includes, our treatment overview sets this out without hiding anything.

Getting the right prescription, what the process looks like

Because these are prescription-only medicines, the legal route to either injection is a clinical assessment followed by a decision by a GPhC-registered prescriber. There is no shortcut around that, and any seller offering these medicines without one is operating outside UK law, the MHRA has been explicit about the risks of counterfeit pens and unregulated supply.

At nume, a real clinician reads your consultation the same day. Not software, not a screening algorithm, a GPhC-registered Independent Prescriber reviews your answers, your weight verification, and your medical history before deciding whether treatment is appropriate and, if so, which medicine fits your situation. If you order before 12pm on a working day and your prescription is confirmed, your treatment is dispatched that afternoon via DPD. It arrives the next working day in plain, unbranded packaging, the pen itself sealed and cold-packed, exactly as dispensed.

Questions about how the process works or what to expect during titration are exactly what our prescribers are there for. If you'd like to understand how effective the weight loss injection is likely to be for you personally, including what realistic outcomes look like beyond the trial averages, that page covers it in detail. When you're ready, and if you're still deciding what the best weight loss injection might be for your circumstances, our prescribers can help you work through that during your consultation, check your eligibility with a free consultation, there's no obligation and no fee to complete it.