The SURMOUNT study on tirzepatide: what the trial evidence actually shows

What SURMOUNT-1 measured and why its numbers matter

Clinical trials for weight-loss medicines typically report one of two things: how much weight participants lost in absolute terms, or what percentage of starting body weight disappeared over the study period. SURMOUNT-1 used both. Participants receiving tirzepatide 15mg lost an average of around 20–21% of body weight over 72 weeks; at 10mg the average was roughly 19%, and at 5mg around 15%. Each of these outperformed the placebo arm, where participants lost around 3%.

Those figures come from a randomised, double-blind, placebo-controlled trial published in the New England Journal of Medicine (NEJM, 2022). Participants followed a reduced-calorie diet and increased their physical activity alongside the injection, which reflects the licensed use: tirzepatide works alongside lifestyle changes, not instead of them.

One thing the headline percentages do not show is the spread. Some participants lost considerably more; others less. Trial averages describe populations, not individuals. A prescriber looks at your starting weight, your health history and your response over time, the numbers from SURMOUNT give context, not a personal forecast. If you want a starting point before a consultation, our guide to Summit tirzepatide walks through the main clinical findings in plain language.

SURMOUNT-5 and the semaglutide comparison

SURMOUNT-5, reported in the NEJM in 2025, was notable because it placed tirzepatide and semaglutide 2.4mg alongside each other in the same trial for the first time. Over 72 weeks, tirzepatide produced greater average weight loss than semaglutide 2.4mg in adults with obesity but without type 2 diabetes. The trial involved 751 adults and was open-label, which is an important methodological caveat, participants knew which medicine they were receiving.

NICE's committee noted in TA1026 that indirect comparisons across separate trial programmes also favoured tirzepatide, though the committee acknowledged the limits of cross-trial inference. Newer higher-dose semaglutide data (the 7.2mg maintenance dose approved by the MHRA in early 2026) narrows the gap, with that dose showing around 20.7% average loss over 72 weeks. Both medicines work; which suits a given person is a clinical question, not a trial-ranking exercise. For a deeper look at the methodology and outcomes behind SURMOUNT-5 specifically, the Summit trial tirzepatide page covers the design and findings in detail.

If you want a fuller look at how tirzepatide's trial programme compares with the broader evidence base, the tirzepatide study overview covers the wider SURMOUNT and SURPASS data together.

What the trial participants looked like, and what that means for eligibility

SURMOUNT-1 enrolled adults with a BMI of 30 or above, or a BMI of 27 or above with at least one weight-related condition such as high blood pressure, dyslipidaemia, obstructive sleep apnoea or cardiovascular disease. Participants with type 2 diabetes were excluded from SURMOUNT-1 (that group was studied in the SURPASS programme and in SURMOUNT-2).

The NICE recommendation that followed (published in December 2024 and updated September 2025) sets the NHS bar at a BMI of 35 or above plus at least one weight-related comorbidity, with lower thresholds applying for South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds. Private eligibility, per the licensed SmPC, begins at BMI 30, or 27 with a qualifying condition. Neither route is automatic: a prescriber reviews the whole picture.

For a plain-English read on what the tirzepatide licence covers, including conditions and the BMI thresholds that apply in different settings, that page lays it out without clinical jargon. The SURPASS diabetes trials are covered separately on the SURPASS study page if that programme is what you were searching for.

Side effects seen in the trial and how prescribers approach them

The most common events in SURMOUNT-1 were gastrointestinal: nausea, diarrhoea, vomiting, constipation, indigestion and reflux. These occurred more often in the tirzepatide arms than placebo, were typically mild to moderate, and were most noticeable after starting treatment or after a dose increase. Discontinuation rates due to side effects were higher in the active-treatment groups but remained a minority of participants.

The titration schedule exists partly because of this profile. Treatment starts at 2.5mg (a dose whose job is to let the body adjust, not to drive significant weight loss) and increases in steps guided by the prescriber. The NHS tirzepatide medicines page lists the recognised side effects alongside practical guidance on what to watch for. A question our prescribers hear regularly is whether side effects mean the medicine should be stopped: the answer depends on severity and pattern, which is why aftercare contact matters throughout treatment.

On the cost side, it is worth knowing that private prices for tirzepatide have moved since Eli Lilly revised its UK list price in September 2025. The background on that price change explains what shifted and why market prices vary across providers. Timing also matters practically: orders placed before 12pm on a working day, once clinically approved, are dispatched the same day. If you are planning ahead around a bank holiday, it is worth factoring that into when you order.

If you have read the SURMOUNT evidence and want to talk through whether tirzepatide fits your situation, our prescribers review consultations the same day. Speak to our prescribers through a free consultation, there is no obligation, and the clinical review is thorough before anything is prescribed.