Which weight loss injection is most effective right now?

You've seen the headlines. Here's what the trial data actually shows.

Picture this: you've spent an evening searching, found figures ranging from 5% to 22% weight loss, and you're no longer sure which number is real or which injection it even refers to. That confusion is completely understandable. The field moved fast, the studies use different timelines, and the marketing around these medicines doesn't always separate one product from another.

Let's ground it in what the clinical trials actually measured. Tirzepatide (sold in the UK as Mounjaro) was tested in the SURMOUNT-1 study across 2,539 adults with obesity. At the highest dose (15 mg, once weekly), participants lost an average of roughly 20–21% of their starting body weight over 72 weeks. That figure comes directly from the SURMOUNT-1 paper published in the New England Journal of Medicine. It is not a best-case ceiling; it is the trial average at that dose.

Semaglutide 2.4 mg (Wegovy) produced around 15% average weight loss over 68 weeks in its STEP 1 trial. That is still a clinically meaningful result, well above anything achieved with older weight-loss medicines. The more recently approved 7.2 mg semaglutide dose reported around 20.7% average loss over 72 weeks in trials, bringing it close to tirzepatide's territory. Worth knowing if your consultation is weighing up the two options.

SURMOUNT-5, published in 2025, put them directly against each other: 751 adults, no diabetes, 72 weeks. Tirzepatide produced the greater average reduction. NICE's appraisal of tirzepatide (TA1026) noted that indirect comparisons favour tirzepatide, and SURMOUNT-5 confirmed that picture in a direct trial.

Why 'most effective overall' is the wrong question to ask your prescriber

The trial averages are real, but averages describe populations, not individuals. Some people lose considerably more than the headline figure; some lose less. Response depends on starting weight, metabolic health, how well GI side effects are tolerated at higher doses, and how consistently the medicine is taken alongside dietary changes.

Tirzepatide works on two gut-hormone pathways, GIP and GLP-1. Semaglutide targets GLP-1 alone. Whether that dual action makes a meaningful difference for you specifically is something a prescriber considers alongside your medical history, any other medicines you take, and your preference about injecting versus oral treatment. There is now also an oral semaglutide tablet (Wegovy tablets, approved by the MHRA in June 2026) for people who prefer not to inject at all, you can read about the alternatives to weight loss injections if that applies to you.

A quick practical check worth doing before any consultation: look at whether the pharmacy or clinic you're considering can verify their registration on the GPhC pharmacy register. It takes under a minute and tells you immediately whether prescriptions will be issued legally and medicines sourced through the licensed UK supply chain. For a prescription medicine, that check matters more than the price.

For a closer look at how the two injections stack up across different measures, this comparison of the most effective weight loss injections covers the detail side by side.

Eligibility, side effects, and what to expect from treatment

Both tirzepatide and semaglutide are licensed for adults with a BMI of 30 or above, or 27 or above if at least one weight-related health condition is present, for example, high blood pressure, high cholesterol, type 2 diabetes, or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. A prescriber assesses your full health picture; meeting the BMI criterion is a starting point, not a guarantee of approval.

Side effects with both medicines are dominated by the gastrointestinal: nausea is the most commonly reported, with some people also experiencing loose stools, constipation, or acid reflux, particularly in the first few weeks or after a dose increase. These effects usually settle as the body adjusts. Less commonly, some people experience fatigue, headache, or dizziness. Severe, persistent abdominal pain that spreads to the back is a reason to seek prompt medical help, the MHRA issued a Drug Safety Update on acute pancreatitis in January 2026 as a known but infrequent risk with GLP-1 medicines generally.

Treatment begins at a low starter dose to reduce the chance of side effects, then is titrated by the prescriber over several months. For tirzepatide, that means starting at 2.5 mg and working upward in steps. The top-line trial results reflect the highest maintenance doses, which most people reach after several months of tolerating each step. Realistic expectations matter: early weeks at lower doses are about adjustment, not peak results. If you want a fuller picture of how effective weight loss injections are across the dose range, that page covers the evidence in detail.

Understanding the cost picture is part of planning realistically. If you are still weighing up which option to pursue, our guide to what the most effective weight loss injection is can help you make sense of the options before you speak to a prescriber. What weight loss injections cost in the UK explains the factors that affect the price, including what a legitimate prescription service includes that a low-cost listing often does not.

At nume, a GPhC-registered prescriber personally reviews every consultation, your answers land on a clinician's desk, not in an automated queue. If you're ready to find out whether one of these treatments is clinically suitable for you, check your eligibility with a free consultation. You can also explore how effective the weight loss injection is likely to be for someone in your situation before you begin.