The SURPASS-1 tirzepatide trial: what it found and why it matters

Step 1: understanding who was in the trial and how it was designed

SURPASS-1 enrolled 478 adults with type 2 diabetes who were managing their condition through diet and exercise alone, without any background glucose-lowering medication. That design choice was deliberate. By testing tirzepatide without other diabetes drugs in the picture, researchers could measure its effects cleanly.

Participants were randomised to one of four groups: tirzepatide 5mg, 10mg or 15mg once weekly, or placebo, for 40 weeks. The primary endpoint was the change in HbA1c, the standard blood test that reflects average blood sugar over roughly three months. Secondary endpoints included body weight, fasting glucose and the proportion of participants reaching an HbA1c below 7% (the typical clinical target).

The trial is published in full in The Lancet, and a summary of tirzepatide's evidence base feeds into the NICE technology appraisal TA1026, which recommended tirzepatide for weight management in the UK.

Step 2: what the results actually showed

Every tirzepatide dose outperformed placebo on every key measure. At 40 weeks, participants on 15mg reduced their HbA1c by an average of around 2.1 percentage points from baseline, compared with a modest increase in the placebo group. More than 90% of those on the highest dose reached an HbA1c below 7%.

Body weight fell across all three doses too, not a trivial secondary finding. At 15mg, average weight loss was around 9.5 kg over the 40 weeks. Given that participants were not enrolled specifically for weight management, that figure drew significant attention from the endocrinology community.

A question our prescribers hear most weeks is whether the weight-loss effects seen in the diabetes trials are comparable to those in the dedicated weight-management trials. They are not directly equivalent: SURPASS-1 participants had type 2 diabetes, which affects metabolism differently, and the trial lasted 40 weeks rather than the 72 weeks used in the SURMOUNT weight-management programme. The SURMOUNT-1 results (around 20% average weight reduction at 15mg in adults without diabetes) come from a distinct population and a longer follow-up. If you want to understand tirzepatide's full evidence base across both indications, it helps to keep those populations separate.

Step 3: where SURPASS-1 sits within the wider programme

SURPASS-1 was the starting point, but the programme expanded considerably. The SURPASS trial programme ran six trials in parallel, each asking a different question: how does tirzepatide compare to established diabetes drugs? How does it perform in people with heart disease? What happens when it is added to insulin?

SURPASS-2 compared tirzepatide head-to-head with semaglutide 1mg (the diabetes dose), you can read about the SURPASS-2 results separately. SURPASS-3 looked at tirzepatide against insulin degludec. Each trial added a layer to the picture that regulators and NICE used to assess the medicine's benefit-risk profile.

For people exploring weight management rather than diabetes treatment, the relevant leap is from SURPASS to SURMOUNT. The broader SURPASS study overview covers that transition. And if you're weighing up what Mounjaro costs as a private prescription, our Mounjaro cost page explains what the typical private market looks like and what a legitimate prescription service includes.

Step 4: what SURPASS-1 means if you're considering tirzepatide for weight management

SURPASS-1 is a diabetes trial, not a weight-loss trial, that distinction is worth holding onto. The weight-loss licence for tirzepatide (Mounjaro) rests on the SURMOUNT programme, which enrolled adults with obesity or overweight rather than type 2 diabetes. But SURPASS-1 mattered for weight management indirectly: the weight reductions seen in a diabetes population flagged tirzepatide as unusually active on appetite and energy balance, prompting the separate SURMOUNT programme that ultimately supported the weight-management licence.

In the UK, Mounjaro is licensed for weight management in adults with a BMI of 30 or above, or 27 or above with at least one weight-related condition such as high blood pressure, high cholesterol or obstructive sleep apnoea. BMI alone is not the whole picture, a prescriber reviews your health history, current medicines and other factors before making a clinical decision. You can read more about how Mounjaro works as a weight-management treatment, and our weight-loss treatment overview sets out the broader options a prescriber might consider.

If you're ready to find out whether you're a suitable candidate, checking your eligibility takes a few minutes and is reviewed the same day by a GPhC-registered prescriber at our pharmacy.