SURMOUNT-5: what the head-to-head trial showed
What did the SURMOUNT-5 trial find when comparing tirzepatide and semaglutide?
SURMOUNT-5 compared tirzepatide and semaglutide directly in the same study, the first trial to do so. Across 72 weeks in 751 adults, tirzepatide produced a mean weight reduction of 20.2% against 13.7% for semaglutide, and greater improvements in blood pressure, HbA1c, triglycerides and HDL cholesterol. It was published in the New England Journal of Medicine. The result describes a trial population, not an individual, and does not make one treatment correct for everyone.
Why a head-to-head trial is different
Almost every comparison you will read between these treatments sets two separate trials side by side. That is unreliable, and not slightly so: trial populations differ in starting weight, comorbidities, age distribution and the intensity of the lifestyle programme running alongside. Comparing a 21% figure from one study with a 15% figure from another tells you less than it appears to.
A head-to-head trial removes that problem by putting both treatments in the same study, with the same population, the same endpoints and the same supporting programme. SURMOUNT-5 is the first to do that for these two molecules, which is why it carries more weight than the arithmetic of comparing separate results.
What the trial measured
The design was straightforward, and the details matter when reading the headline figure.
- 751 adultsRandomised to maximum tolerated doses of tirzepatide (10 or 15 mg) or semaglutide (1.7 or 2.4 mg), once weekly.
- 72 weeksLong enough to capture the plateau rather than only the early loss phase.
- BMI 30+, or 27+ with a complicationParticipants did not have type 2 diabetes, which is a population where weight loss figures tend to run higher than in people who do.
- Primary endpointPercentage change in body weight from baseline.
The result, and what sits behind it
Tirzepatide produced a mean reduction of 20.2% against 13.7% for semaglutide, roughly 22.8 kg against 15.0 kg. Tirzepatide also showed greater improvements across cardiometabolic measures: systolic and diastolic blood pressure, HbA1c, fasting insulin, triglycerides and HDL cholesterol.
The mechanistic explanation is the obvious one. Tirzepatide acts on two receptors, GIP and GLP-1; semaglutide acts on GLP-1 alone. The trial is consistent with the dual mechanism doing additional work, though a single trial establishes the size of an effect rather than settling why it occurs.
What the result does not mean
It does not mean tirzepatide is the right treatment for any particular person. A mean is the centre of a distribution: some participants on semaglutide lost more than the tirzepatide average, and some on tirzepatide lost less than the semaglutide average.
It also does not account for tolerability, which is where treatment decisions are frequently made in practice. Someone who cannot tolerate the gastrointestinal effects at a higher tirzepatide dose does better on a treatment they will actually continue.
And it says nothing about maintenance. Both treatments work while they are being taken; the trial did not measure what happens to either group after stopping. That question, what holds the result, is the one this site treats as the whole point of a programme.
Common questions
Is Mounjaro better than Wegovy according to SURMOUNT-5?
On the trial's primary endpoint, tirzepatide produced greater mean weight reduction: 20.2% against 13.7% over 72 weeks. That is a population average rather than a prediction for an individual, and it does not account for tolerability or existing conditions. Which treatment is appropriate is a clinical decision.
Was SURMOUNT-5 peer reviewed?
Yes. The results were published in the New England Journal of Medicine, which is the distinction between a trial result and a press release. Peer-reviewed publication is what allows the design and analysis to be scrutinised rather than only the headline number.
Why were people with diabetes excluded?
Participants did not have type 2 diabetes. That matters when comparing figures across studies, because weight reduction in trials tends to be smaller in populations with diabetes. A result from a non-diabetic population should not be read across to a diabetic one.
Sources
- SURMOUNT-5: greater loss of weight and waist circumference with tirzepatide than semaglutide · American College of Cardiology
- Tirzepatide demonstrates superior weight loss to semaglutide in SURMOUNT-5 · Applied Clinical Trials
- Tirzepatide for managing overweight and obesity (TA1026) · NICE
Awaiting sign-off by a named clinician.
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