In the first direct clinical comparison of the two most talked-about weight-loss injections, tirzepatide, the active ingredient in Eli Lilly's Mounjaro and Zepbound, outperformed semaglutide, the drug behind Novo Nordisk's Ozempic and Wegovy.
In the 751-patient SURMOUNT-5 trial published in the New England Journal of Medicine, adults on tirzepatide lost an average of 20.2% of their body weight over 72 weeks.
Those on semaglutide lost 13.7%. The gap works out to roughly 47% greater weight loss on tirzepatide.
A 2026 systematic review and meta-analysis in Clinical Obesity pooled 10 head-to-head studies covering more than 41,000 people and reached the same verdict.
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Tirzepatide produced about four extra percentage points of weight loss on average, and made patients notably more likely to hit key thresholds such as at least 15% or 20% of body weight lost.
Why one drug pulls ahead
The likely reason is that the two drugs work through slightly different biology.
Semaglutide activates a single gut-hormone receptor called GLP-1, which curbs appetite and slows stomach emptying.
Tirzepatide activates that same receptor plus a second one, GIP. In the words of the 2026 review, tirzepatide combines GLP-1 receptor activation with GIP receptor agonism, engaging what the authors describe as complementary neuroendocrine pathways.
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In SURMOUNT-5, more than 31% of people on tirzepatide lost at least a quarter of their body weight over 72 weeks. On semaglutide, 16.1% reached that mark.
Tirzepatide also trimmed waist circumference more, cutting about 18.4 cm on average versus 13.0 cm on semaglutide, according to a summary of the trial from the American College of Cardiology.
What the safety picture looks like
Both drugs carry the familiar side-effect profile of the GLP-1 class: nausea, vomiting, diarrhea and constipation, mostly mild to moderate and typically worst while the dose is being stepped up.
In SURMOUNT-5, more people stopped semaglutide because of gut side effects (5.6%) than stopped tirzepatide (2.7%).
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The 2026 review found overall rates of gut side effects were similar across the two drugs, with no meaningful difference in the rate of dropping out because of side effects. It also flagged one caution, noting that serious adverse events were reported more often with tirzepatide.
What to do with the numbers
Both drugs are prescription-only weekly injections, and neither should be started without a doctor's assessment.
Insurance coverage, out-of-pocket cost, personal tolerance and other health conditions such as type 2 diabetes, heart disease or kidney disease all matter, and can point one way or the other.
The 47% figure is an average across a large group. For any individual person, the difference in the mirror after a year could be larger, smaller or invisible.
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Tirzepatide is sold as Zepbound for chronic weight management and as Mounjaro for type 2 diabetes. Semaglutide is sold as Wegovy for weight management and as Ozempic for diabetes. All four are given as a once-weekly injection under the skin.
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This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.