What did the SURMOUNT-1 trial find about tirzepatide?

Who was in the trial, and why does that matter?

SURMOUNT-1 enrolled adults with a BMI of 30 or above, or 27 and above with at least one weight-related condition such as hypertension, dyslipidaemia, obstructive sleep apnoea, or cardiovascular disease. Crucially, participants with type 2 diabetes were excluded, that population was studied separately in the SURMOUNT-2 trial. This distinction shapes how the results translate to clinical practice: the evidence base for tirzepatide in people living with obesity but without diabetes is particularly strong and well-characterised.

All participants also received intensive lifestyle support throughout the trial: counselling on a reduced-calorie diet and guidance on increased physical activity. That context is worth holding onto. The headline weight-loss figures reflect tirzepatide added to meaningful lifestyle change, not the medicine alone. Our overview of the SURMOUNT trial programme sets this study alongside the others for a fuller picture.

One thing the trial does not tell you is how any individual person will respond. Biology, baseline weight, adherence, and health history all vary. A prescriber reviewing your full picture is the right person to set expectations, and if you want background on the broader programme, our tirzepatide SURMOUNT page covers the context behind these studies in one place.

What did the numbers actually show?

At 72 weeks, participants on 5mg tirzepatide lost an average of around 15% of their body weight. Those on 10mg lost roughly 19.5%, and those on 15mg achieved average reductions of between 20% and 22.5% depending on the analysis. The placebo group, also receiving lifestyle support, lost about 3% on average. The gap is substantial by any clinical standard.

Beyond the headline percentage, SURMOUNT-1 reported that a large proportion of participants on the higher doses lost at least 20% of their body weight, a threshold that had rarely been crossed in trials of any medical weight-management treatment before tirzepatide. These figures are what sits behind NICE's decision to recommend tirzepatide in TA1026, published in December 2024.

For broader context on how these results compare across the SURMOUNT programme, the tirzepatide weight-loss percentages page brings together the figures from each study.

What side effects did SURMOUNT-1 record?

The most common adverse events were gastrointestinal: nausea, diarrhoea, vomiting, and constipation. These were more frequent at the point of starting treatment or after a dose increase, and for most participants they were mild to moderate and settled within a couple of weeks. Discontinuation due to side effects was higher in the tirzepatide groups than placebo, though most people in all three active-dose groups completed the trial.

Tirzepatide treatment starts at 2.5mg, a dose whose job is to let your body adjust, not to drive the majority of weight loss. The prescriber then steps the dose up at roughly four-weekly intervals, giving the body time to settle at each stage. If side effects become difficult at a given dose, titration can be paused; that decision sits with the prescriber, not a protocol.

The SURMOUNT-4 trial later explored what happens when tirzepatide is stopped after sustained use, a question the original 72-week results naturally raised.

How does SURMOUNT-1 connect to eligibility for treatment today?

The SURMOUNT-1 population maps closely to the private licensed eligibility criteria for tirzepatide in the UK: adults with a BMI of 30 or above, or 27 and above with a qualifying weight-related condition. NICE's recommendation in TA1026 applies a higher bar for NHS access (BMI 35 or above with at least one comorbidity) but private prescribers can assess within the licensed indication, which is broader.

Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance, and the prescriber assesses the whole picture rather than BMI alone. If you're weighing up whether tirzepatide might be relevant for you, the Mounjaro (tirzepatide) treatment page covers eligibility in detail, and the cost context page explains what a private prescription typically involves.

Tirzepatide is sold in the UK under the brand name Mounjaro, supplied as a once-weekly pre-filled KwikPen, a compact device that clicks and locks after use. Yours would arrive in plain, unbranded packaging via tracked DPD delivery, refrigerated for the journey. At nume, every consultation is reviewed the same day by a GPhC-registered Independent Prescriber before any treatment is approved or dispatched.

If you'd like to explore whether tirzepatide is suitable for your situation, you can start a free consultation with our clinical team.