A randomised comparison

Participants were assigned to the maximum tolerated dose of tirzepatide, 10 mg or 15 mg weekly, or semaglutide, 1.7 mg or 2.4 mg weekly, for 72 weeks. The phase 3b study was open-label, so participants and clinicians knew which treatment each person had. Randomisation makes the groups comparable. Open-label treatment still affects how you read adherence, reported symptoms and other behaviour.

The main result

The least-squares mean percentage weight change at week 72 was −20.2% with tirzepatide and −13.7% with semaglutide. Tirzepatide gave greater average weight loss under the study protocol. Subtracting the means gives a gap of 6.5 percentage points. Individual responses varied, so this does not mean everyone lost an extra 6.5% of their own starting weight.

Why a head-to-head trial beats separate rankings

In a head-to-head study, both groups join under one protocol, are assessed over the same period and are analysed in the same framework. This removes several problems you meet when comparing unrelated trials. Some uncertainty remains, and the result does not apply to everyone. The study still had eligibility criteria, set doses and its own approach to people stopping treatment and to missing data.

What SURMOUNT-5 did not test

The trial did not test Wegovy tablets or semaglutide 7.2 mg. SURMOUNT-5 also did not compare starting doses or measure the result in people with type 2 diabetes. Claiming tirzepatide beats every current and future semaglutide regimen goes beyond the evidence. As the product range changes, the original trial still stands. Comparisons need precise wording.

Average effect is one factor among several

You may tolerate one medicine better, have a relevant licensed indication, prefer a particular device or need a cheaper long-term plan. A difference in weight loss tells you nothing about other health outcomes. Your clinician should combine the comparative evidence with your medical history and priorities. The larger average percentage alone should not decide your treatment.

Discussing a possible change

If you are doing well on one treatment, SURMOUNT-5 gives you reason for an informed conversation. The trial is no instruction to switch at once. Bring a record of benefits, symptoms and treatment burden. Ask what a switch should improve and how your team will measure the change. Get a written transition plan, because trial maintenance doses are not interchangeable personal starting doses.

Relative and percentage-point differences

Two comparisons can both be correct and sound quite different. The gap between mean losses of 20.2% and 13.7% is 6.5 percentage points. Expressed against the smaller figure, the same gap becomes a much larger-sounding percentage. Neither calculation predicts your result, and the relative version can hide the real changes in starting weight. Report both original trial means with the population and duration, so readers grasp the finding without unpicking a promotional headline.

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-5: original trial publication nejm.org
  2. Mounjaro: UK prescribing information medicines.org.uk
  3. Wegovy injection: UK prescribing information medicines.org.uk

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