The SURMOUNT clinical trial programme is the landmark series of studies that established tirzepatide (Mounjaro) as a leading weight management medication. These trials provided the evidence base for regulatory approvals worldwide, including in the UK. Here is a summary of the key findings.
SURMOUNT-1: The Flagship Trial
SURMOUNT-1 was a randomised, double-blind, placebo-controlled trial involving 2,539 adults with obesity or overweight with at least one weight-related comorbidity (excluding diabetes). Key results at 72 weeks:
- 5 mg dose: 15.0% average body weight reduction
- 10 mg dose: 19.5% average body weight reduction
- 15 mg dose: 20.9% average body weight reduction
- Placebo: 3.1% body weight reduction
Over one-third of participants on the highest dose achieved weight loss of 25% or more — results previously only seen with bariatric surgery.
SURMOUNT-2: Tirzepatide in Type 2 Diabetes
This trial focused on adults with both obesity and type 2 diabetes, a population that typically finds weight loss more difficult. At 72 weeks, participants achieved 12.8% (10 mg) and 14.7% (15 mg) weight loss, alongside significant HbA1c reductions. These results demonstrated that tirzepatide is effective even in this harder-to-treat population.
SURMOUNT-3: Intensive Lifestyle Intervention
SURMOUNT-3 investigated tirzepatide following a 12-week intensive lifestyle intervention (calorie restriction). Participants who had already lost weight through lifestyle changes achieved a further 18.4% weight loss with tirzepatide 15 mg, compared to 2.5% regain with placebo. This supports the use of Mounjaro as a maintenance therapy after initial lifestyle-based weight loss.
SURMOUNT-4: Weight Regain After Discontinuation
This pivotal trial addressed the question of what happens when treatment stops. After 36 weeks of open-label tirzepatide, participants who switched to placebo regained approximately 14% of their body weight over the following 52 weeks, while those continuing treatment maintained or further reduced their weight. This has important implications for treatment duration discussions with UK prescribers.
Safety Across the Programme
Gastrointestinal events were the most common side effects across all SURMOUNT trials:
- Nausea: 24-33% (usually mild to moderate and transient)
- Diarrhoea: 17-23%
- Constipation: 11-17%
- Treatment discontinuation due to adverse events: 4-7%
What This Means for UK Patients
The SURMOUNT programme provided NICE and the MHRA with the evidence needed to approve Mounjaro for weight management in the UK. The consistency of results across diverse populations gives UK patients and clinicians confidence in the treatment's efficacy. Discuss these findings with your prescriber to understand how they apply to your individual circumstances.