Inside The SURMOUNT Trials

The SURMOUNT programme is what turned tirzepatide from a strong diabetes medication into a category-defining weight management drug. Five trials between 2020 and 2024, in different populations and with different designs, all pointing in the same direction: tirzepatide produces mean weight reduction substantially larger than any pharmaceutical that came before. The October 2025 SAHPRA approval for chronic weight management rested on this evidence.

Here is what each trial actually measured and what it showed.

SURMOUNT-1: Adults With Obesity Without Diabetes

What it tested: Tirzepatide at three doses (5, 10, 15 mg) vs placebo in adults with obesity (BMI 30+) or overweight (BMI 27+) with at least one weight related condition, but without type 2 diabetes. About 2,539 participants. 72 weeks.

Headline finding: Mean weight reduction at 72 weeks was 15.0 percent at 5 mg, 19.5 percent at 10 mg, and 20.9 percent at 15 mg, compared to 3.1 percent on placebo. Almost 90 percent of patients on tirzepatide 10 or 15 mg achieved at least 5 percent weight reduction. About half achieved 20 percent or more.

Why it mattered: The headline trial. Demonstrated the magnitude of effect in a population without diabetes, where weight reduction tends to be larger than in diabetic populations. The 20 percent average effect at maximum dose was historic, exceeding the 15 percent seen with semaglutide and far exceeding older medications.

SURMOUNT-2: Adults With Obesity And Type 2 Diabetes

What it tested: Tirzepatide at 10 and 15 mg vs placebo in adults with obesity and type 2 diabetes. About 938 participants. 72 weeks.

Headline finding: Mean weight reduction of 12.8 percent at 10 mg and 14.7 percent at 15 mg, compared to 3.2 percent on placebo. HbA1c also reduced substantially in tirzepatide arms.

Why it mattered: Confirmed that the weight management effect extends to patients with diabetes, although somewhat smaller than in non-diabetic populations. Important because the indication being sought was specifically inclusive of patients with type 2 diabetes.

SURMOUNT-3: After Intensive Lifestyle Intervention

What it tested: A 12 week intensive lifestyle intervention, followed by randomisation to either tirzepatide at maximum tolerated dose or placebo for 72 weeks. About 806 participants total.

Headline finding: Participants who had completed the lifestyle intervention and lost some weight then continued to lose substantially more on tirzepatide (an additional 18.4 percent on top of lifestyle reduction) compared to small regain on placebo.

Why it mattered: Modelled what many real world patients experience. Most people considering medication for weight management have already tried lifestyle changes. SURMOUNT-3 showed that tirzepatide adds substantial benefit even after lifestyle interventions have done their work.

SURMOUNT-4: The Withdrawal Trial

What it tested: All participants received tirzepatide for 36 weeks. They were then randomised to either continue tirzepatide or switch to placebo for another 52 weeks. About 670 participants reached the randomisation stage.

Headline finding: Participants who continued tirzepatide continued to lose weight, reaching a total of about 25.3 percent reduction by the end. Participants who switched to placebo regained substantial weight, ending at about 9.9 percent total reduction (having regained much of what was lost during the open-label phase).

Why it mattered: Demonstrated definitively that the weight management effect is dependent on continued medication. Stopping leads to regain. This established the medication as appropriate for long term chronic use rather than a finite course of treatment.

SURMOUNT-5: Head To Head Vs Semaglutide

What it tested: Tirzepatide at maximum tolerated dose (up to 15 mg) vs semaglutide 2.4 mg in adults with obesity. About 751 participants. 72 weeks.

Headline finding: Mean weight reduction at 72 weeks was 20.2 percent on tirzepatide vs 13.7 percent on semaglutide. Tirzepatide also produced larger reductions in waist circumference, blood pressure, and lipid markers.

Why it mattered: The decisive head to head comparison that the field had been waiting for. SURMOUNT-5 confirmed in a direct comparison what the separate SURMOUNT-1 and STEP-1 trials had suggested: tirzepatide produces stronger average weight reduction than the maximum semaglutide dose. The 6.5 percentage point gap is the clearest single piece of evidence for the value of the dual receptor mechanism.

SURMOUNT-OSA: Obstructive Sleep Apnoea

Beyond the main programme, several SURMOUNT trials looked at specific conditions where weight management would provide benefit. SURMOUNT-OSA studied tirzepatide in adults with obesity and moderate to severe obstructive sleep apnoea. The result was substantial reduction in apnoea severity alongside weight loss. For some patients, severity reduced enough to no longer meet criteria for ongoing CPAP therapy.

This and related sub-studies (SURMOUNT in cardiovascular outcomes, NAFLD, knee osteoarthritis) have expanded the clinical case beyond pure weight reduction.

The Pattern Across The Programme

Five main trials, varied populations and designs, consistent direction:

• Mean weight reduction of 15 to 21 percent at maximum dose, depending on population
• Around half of patients on maximum dose achieve 20 percent or more reduction
• Effect extends to diabetic populations although somewhat smaller
• Effect is dose dependent. Larger doses produce larger average effects
• Effect dependent on continued treatment. Stopping causes regain
• Effect adds to lifestyle intervention rather than replacing it
• Effect superior to semaglutide in direct comparison
• Side effect profile consistent with GLP-1 class

What These Numbers Are Not

These are average outcomes in clinical trial populations using the medication alongside structured lifestyle support. They are not promises for any specific individual. Individual response varies considerably. Some people achieve much more, some much less. The trials estimate the average effect of treatment, not the certain effect for any one person.

What the trials reliably show is the direction and approximate magnitude. The actual goal of treatment for any individual patient is decided between the patient and the treating doctor based on starting health, comorbidities, and what is sustainable.

How To Read The Programme

SURMOUNT confirmed three things that matter for SA patients:

1. Mounjaro produces strong average weight reduction. Not 5 percent. Not 10 percent. Around 20 percent at maximum dose. This is a different magnitude of effect from older medications.
2. The benefit requires continued treatment. Stopping causes regain. The medication is chronic treatment for a chronic condition.
3. The benefit extends beyond weight on the scale. Cardiovascular markers, sleep apnoea, liver fat, joint pain all improve alongside weight reduction.