What the UK weight loss injection trials actually found

Step 1: what the two main trial programmes set out to do

Both the SURMOUNT and STEP programmes were large, randomised, placebo-controlled trials, the standard the MHRA and NICE require before recommending a medicine for widespread use. Participants in each trial were adults living with obesity or overweight plus a weight-related condition; none had type 2 diabetes in the primary weight-management arms. All participants received advice on reducing calorie intake and increasing physical activity alongside either the active injection or a placebo.

SURMOUNT-1, the pivotal trial for tirzepatide, ran for 72 weeks and enrolled 2,539 people. STEP 1, the equivalent for semaglutide at the 2.4mg weekly dose, ran for 68 weeks with 1,961 participants. Both were designed to measure percentage body-weight reduction as the primary outcome, alongside secondary measures including waist circumference, blood pressure and quality of life. The SURMOUNT-1 results published in the New England Journal of Medicine set a new benchmark for a non-surgical intervention.

If you want a plain summary of what these medicines are and how they work, our overview of the weight loss injection covers the basics in everyday language.

Step 2: what the results actually showed, and what the numbers mean

At the highest tirzepatide dose studied (15mg), the average body-weight reduction across all SURMOUNT-1 participants was around 20–21%. At 10mg the average was roughly 19%, and at 5mg around 15%. These are averages across the entire group: some participants lost considerably more, others less. A placebo group following the same lifestyle advice lost about 3% on average over the same period. The gap between those figures (not the absolute number) is what matters clinically.

STEP 1 reported an average reduction of around 15% at the semaglutide 2.4mg maintenance dose over 68 weeks, against roughly 2.4% in the placebo group. NICE's appraisal of both medicines took these figures into account; you can read the full analysis in NICE technology appraisal TA1026 for tirzepatide.

A later head-to-head trial, SURMOUNT-5, directly compared tirzepatide against semaglutide 2.4mg in adults with obesity but without diabetes. Published in the New England Journal of Medicine in 2025, it reported greater average weight loss with tirzepatide, the first robust direct comparison between the two medicines. NICE's committee discussion on TA1026 noted that indirect comparisons also favoured tirzepatide, while acknowledging that semaglutide's newer 7.2mg dose narrows the gap.

It is worth being clear that trial averages do not predict individual results. How much weight any person loses depends on the dose reached, how long they stay on treatment, their diet and activity, and their individual biology. No result is guaranteed.

Step 3: how UK regulators used the trial data to set eligibility

The MHRA granted UK marketing authorisation for both medicines based on the trial programmes, then NICE translated that evidence into NHS prescribing recommendations. For tirzepatide, NICE recommends NHS treatment for adults with a BMI of 35 or above and at least one weight-related comorbidity, with BMI thresholds set 2.5 kg/m² lower for South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds. For semaglutide (Wegovy), NICE's recommendation under TA875 restricts NHS use to a maximum of two years within a specialist weight management service.

Private prescribing follows the licensed indications from the SmPC rather than the NHS-specific NICE criteria. That means adults with a BMI of 30 or above, or 27 or above with a qualifying condition such as high blood pressure or obstructive sleep apnoea, may be clinically eligible for a private prescription, subject to a prescriber's assessment of their individual circumstances. The NHS England guidance on weight management injections sets out the NHS route in full.

For a sense of where private treatment sits relative to NHS access, the treatment options page explains what a consultation through a regulated online pharmacy involves. Separately, if cost is a consideration, the context around injection pricing is worth reading before comparing providers.

Step 4: safety signals from the trials and what MHRA monitoring adds

Side effects in both trial programmes were predominantly gastrointestinal: nausea, diarrhoea, constipation and vomiting were the most commonly reported, particularly in the weeks after starting or increasing a dose. Most participants described these as mild to moderate, and rates fell as treatment continued. A minority stopped treatment due to side effects.

Because both medicines carry a Black Triangle (▼) designation, the MHRA requires ongoing pharmacovigilance beyond the trials. On 29 January 2026, the MHRA issued a Drug Safety Update specifically highlighting acute pancreatitis as an infrequent but serious known risk with GLP-1 medicines: severe, persistent stomach pain that may spread to the back, with or without vomiting, warrants urgent medical attention. Suspected side effects can be reported directly at the MHRA's Yellow Card scheme.

Treatment is always started at the lowest dose and increased by a prescriber over several weeks, precisely because of this side-effect profile. The dose schedule is not something to adjust without clinical input; missed doses and timing questions are for your prescriber and the Patient Information Leaflet. Our clinical team profile at the prescribers page gives a sense of the clinical oversight behind every consultation at nume.

If you are considering treatment, this guide on sourcing injections safely covers what to check before choosing a provider, and if you have already been exploring options you may also find it useful to read about what the weight loss injection r option involves or to look into whether the triple g weight loss injection suits your circumstances. Once a prescription is approved and dispatched, the pen arrives the next working day in a plain, unbranded box with DPD tracking, stored conditions are maintained throughout. Speak to our prescribers to find out whether you are clinically suitable.