The trial question

The phase 3 trial randomised 2,539 adults to weekly tirzepatide at target doses of 5 mg, 10 mg or 15 mg, or to placebo, for 72 weeks. Doses rose over a 20-week escalation period. Everyone also had a lifestyle programme. The design asks whether adding the study medicine improves results over placebo under the same conditions. SURMOUNT-1 did not compare tirzepatide with Wegovy.

The main weight result

Under the treatment-regimen estimand, mean body-weight change was −15.0% at 5 mg, −19.5% at 10 mg and −20.9% at 15 mg, against −3.1% with placebo. The estimand defines the effect the analysis sets out to measure, including how it handles events such as people stopping treatment. These figures are group averages at one time point. No dose guarantees a fixed amount of weight loss.

What percentage weight change means

A percentage measures the change against starting weight. You can compare people of different sizes more fairly this way than with kilograms alone. The figure is not the share of body fat lost, and it does not separate fat from lean tissue. For your own care, read weight alongside nutrition, strength, symptoms and other health measures. No trial endpoint captures every part of wellbeing.

Why the placebo group matters

The placebo group accounts for the lifestyle programme, contact with researchers and other changes over time. So you cannot credit the whole weight change in the tirzepatide groups to the medicine alone. The gap between groups and the change within one group are two different numbers. Both are useful. Label each one clearly and never swap them in a headline.

What SURMOUNT-1 cannot tell you

SURMOUNT-1 does not show the outcome for someone with type 2 diabetes, a child, a pregnant person or someone using an unregulated product. The trial cannot tell you which dose you will tolerate. The 72-week main result also does not prove the benefit lasts unchanged after you stop. Later studies and extensions answer further questions, and each needs naming on its own.

Using the result in a consultation

Ask how closely you match the trial population and which health outcome matters most to you. Your clinician should cover tolerability, dose escalation, monitoring and a realistic review plan. Do not raise your dose to chase the highest study average. Evidence helps when it shapes expectations and decisions. A population result turns misleading once people treat the figure as a target every patient must hit.

Reading a percentage in kilograms

Take a starting weight of 100 kg as an example. A 15% reduction means 15 kg lost and a new weight of 85 kg. The sum shows how the measure works and forecasts nobody's result. At a different starting weight, the same percentage means a different number of kilograms. Adverts should not turn a trial average into a promised amount for everyone. A pounds-lost headline also needs the study's starting weight before the number means anything.

Sources

Your own medicine leaflet and prescriber come first. A source from another country does not mean the same rule applies in the UK.

  1. SURMOUNT-1: original trial publication nejm.org
  2. Mounjaro: UK prescribing information medicines.org.uk

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