Tirzepatide weight loss percentage across the SURMOUNT trials: what the evidence shows

What SURMOUNT-1 actually measured, and why the percentages vary

The headline figures from SURMOUNT-1 are striking, but they need a little unpacking to be genuinely useful. The trial, published in the New England Journal of Medicine, randomised 2,539 adults with a BMI of 30 or above (or 27 or above with at least one weight-related condition) who did not have type 2 diabetes. Participants received once-weekly tirzepatide at 5mg, 10mg, or 15mg, or a placebo, alongside a reduced-calorie diet and increased physical activity, for 72 weeks.

Average weight reductions were approximately 15% at 5mg, 19.5% at 10mg, and 20–21% at 15mg, compared with around 3% in the placebo group, and you can read a detailed breakdown of how those tirzepatide weight loss percentages were recorded across the trials if you want to go deeper into the numbers. Some analyses of the 15mg arm, which looked at participants who adhered fully to treatment, reached approximately 22.5%. The variation matters because quoting a single figure without specifying the dose or the analysis method can mislead. A person starting at 2.5mg (the standard tolerability dose) is not yet at a therapeutic level; the results above reflect maintenance doses reached after a gradual titration period.

It is also worth noting that percentage weight loss is an average across a trial population. Individual results spread either side of that average based on starting weight, metabolic profile, dietary changes, and activity. That is not a caveat to brush past, it is why clinical oversight throughout treatment exists.

The SURMOUNT trials beyond SURMOUNT-1: different populations, different percentages

SURMOUNT-1 is the most-quoted trial because it studied adults without diabetes and produced the largest percentage reductions. But the SURMOUNT programme covers a broader range of people, and if you want to understand the full scope of that evidence, our detailed look at the tirzepatide SURMOUNT trials covers each study in the programme and what it found.

SURMOUNT-2 studied adults with type 2 diabetes, where average weight reductions at 15mg were around 15%, lower than in SURMOUNT-1, which is consistent with what researchers expected given that diabetes affects the body's response to GLP-1 and GIP receptor stimulation. The SURMOUNT-OSA trial looked at tirzepatide in adults with obesity and obstructive sleep apnoea; findings on weight reduction in that population contributed to discussions about tirzepatide's effects beyond glycaemic control. Each sub-trial adds a layer to the clinical picture, and NICE's appraisal considered the programme as a whole rather than a single data point.

One practical habit worth building: if you see a tirzepatide weight loss percentage quoted anywhere online, check which trial it comes from and which dose arm. Thirty seconds with the source usually clarifies whether the figure applies to the population and dose you are actually asking about.

SURMOUNT-5 and the head-to-head comparison with semaglutide

Until SURMOUNT-5, comparisons between tirzepatide and semaglutide 2.4mg relied on indirect cross-trial analysis, always a methodological compromise. SURMOUNT-5, published in the New England Journal of Medicine in 2025, ran a 72-week open-label head-to-head in 751 adults with obesity but without diabetes.

Tirzepatide produced a greater average weight reduction than semaglutide 2.4mg in that trial. NICE's committee discussion notes, alongside TA1026's indirect comparisons, also favour tirzepatide. This is the evidence base that positions tirzepatide as producing the largest average reductions among currently licensed weight-management medicines in the UK. For context on how those percentage figures translate to real-world Mounjaro use, the picture is similar but worth reading separately, since real-world adherence typically falls short of trial conditions.

Wegovy's higher 7.2mg dose, approved in the UK in early 2026, narrows the gap; trial data at that dose showed around 20.7% average weight loss over 72 weeks, approaching the tirzepatide 15mg result. Which medicine suits a particular person is a clinical decision that depends on their history, tolerability, and circumstances, not a league table.

What NICE concluded, and what it means for accessing tirzepatide

NICE's recommendation (TA1026), published in December 2024 and updated in September 2025, cites the scale of tirzepatide's weight reductions as central to its positive conclusion. NICE recommends tirzepatide for adults with a BMI of 35 or above and at least one weight-related comorbidity; thresholds are 2.5 kg/m² lower for South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds. NHS access is currently being phased in, with the strictest eligibility criteria applying first and broader cohorts opening through 2026 and 2027.

For people who do not meet NHS thresholds, or who prefer not to wait, private prescription through a regulated UK pharmacy is the legal route. The same prescription-only requirement applies regardless: a clinician must assess your suitability. You can read more about the tirzepatide SURMOUNT evidence that informed its licensed use in the UK and the clinical considerations involved, or explore weight management treatment options more broadly.

If the trial evidence has convinced you that tirzepatide is worth exploring for your own situation, the right next step is a clinical conversation. Speak to our prescribers, a GPhC-registered Independent Prescriber reads every consultation personally, usually the same day.