What the UK tirzepatide trials found — and what it means for you

Step 1: Understanding the trial design before reading the numbers

The SURMOUNT programme was a series of phase 3 randomised controlled trials designed to test tirzepatide across different populations. SURMOUNT-1 is the most cited for weight management: it enrolled 2,539 adults with obesity or overweight plus at least one weight-related condition, but without type 2 diabetes. Participants were assigned to weekly injections of 5 mg, 10 mg or 15 mg tirzepatide, or placebo, alongside a reduced-calorie diet and increased physical activity. The trial ran for 72 weeks.

A question our prescribers hear most weeks is whether the trial participants were similar to ordinary patients in the UK. The SURMOUNT-1 cohort had an average starting weight of around 104 kg and an average BMI of approximately 38. That is broadly comparable to the adults who seek treatment through services like ours. The trial excluded people with active serious conditions and those on certain other medicines, which is why a full clinical assessment matters before starting treatment, the trial's entry criteria are a useful template for understanding when tirzepatide is and is not appropriate.

SURMOUNT-2 tested tirzepatide specifically in adults with type 2 diabetes and obesity, while later arms of the programme examined cardiovascular outcomes and other comorbidities. The full SURMOUNT trial programme is worth understanding as a whole if you want to see how the evidence stacks up across different patient groups.

Step 2: The headline results, dose by dose

At 72 weeks in SURMOUNT-1, average weight reduction was around 15% at 5 mg, around 19.5% at 10 mg, and around 20–21% at 15 mg, compared with roughly 3% for placebo. The 15 mg result was particularly notable: it placed tirzepatide ahead of every weight-loss medicine that had been licensed in the UK at that point. Those figures are published in SURMOUNT-1 in the New England Journal of Medicine and were a central part of NICE's review.

It is worth being clear about what averages mean. Some participants lost considerably more; others lost less. Weight loss on tirzepatide is real and, for many people, substantial, but it is not uniform, and it depends on dose, duration, adherence, diet, activity and individual biology. The trial ran alongside structured lifestyle support, which is also part of the licensed indication: tirzepatide is authorised as an adjunct to a reduced-calorie diet and increased physical activity, not as a standalone intervention.

For context on how tirzepatide's results compare with those of semaglutide across both injection and newer oral formats, the weight-loss treatment overview sets out the licensed options side by side.

Step 3: The SURMOUNT-5 head-to-head and what NICE concluded

SURMOUNT-5, published in the New England Journal of Medicine in 2025, was the first large randomised trial to compare tirzepatide directly with semaglutide 2.4 mg (Wegovy). It enrolled 751 adults with obesity and no diabetes and ran for 72 weeks. Tirzepatide produced greater average weight loss. That finding informed NICE's committee discussions and sits alongside indirect comparisons that also favoured tirzepatide. NICE published its appraisal (TA1026) in December 2024, updated in September 2025, recommending tirzepatide for adults with a BMI of 35 or above and at least one weight-related comorbidity. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance.

NICE also noted that if less than 5% of body weight is lost after six months at the highest tolerated dose, continuing treatment should be reviewed. That stopping rule is part of how NHS commissioners manage the medicine responsibly. For those accessing tirzepatide privately, the same clinical logic applies: a prescriber should assess progress at each repeat, and you can read more about how this works in practice on our Mounjaro trial page.

The SURPASS trial programme covers tirzepatide's evidence in type 2 diabetes, which is a separate licensed indication. And if you are specifically interested in cardiovascular outcome data, the tirzepatide cardiovascular trial page covers that arm of the evidence.

Step 4: From trial data to a real clinical decision

Trial results answer a population-level question: on average, across thousands of people, what does this medicine do? A prescriber answers a different question: is this the right medicine for this person, at this point, given their weight, health history, other medicines and circumstances?

Tirzepatide is a Black Triangle medicine (▼), meaning the MHRA maintains additional monitoring as post-marketing data accumulates. That is not a reason for alarm (most new medicines carry the designation for a period) but it does mean reporting any unexpected side effects via the Yellow Card scheme is genuinely useful, not just a formality.

The most common side effects seen in the trials were gastrointestinal: nausea, diarrhoea, constipation, vomiting and reflux were reported more often with tirzepatide than with placebo. These effects were typically most noticeable after starting or after a dose increase, and in many cases settled within the first couple of weeks. For more on individual situations (including the kidney and liver disease populations studied in subsequent trials) the tirzepatide CKD trial page covers that evidence specifically.

If you would like to explore whether tirzepatide is clinically suitable for you, check your eligibility with our prescribers, every consultation is reviewed the same day by a GPhC-registered Independent Prescriber.