SURMOUNT-5: tirzepatide's efficacy edge over semaglutide, confirmed
The first head-to-head RCT of tirzepatide vs semaglutide for obesity reported 20.2% vs 13.7% mean weight loss at 72 weeks. What the trial settles, and what it doesn't.
SURMOUNT-5, published in the New England Journal of Medicine in May 2025, was the first randomized controlled trial directly comparing Eli Lilly's tirzepatide (Zepbound) with Novo Nordisk's semaglutide (Wegovy) for obesity. The headline result: tirzepatide 15 mg produced 20.2% mean weight loss at 72 weeks vs Wegovy 2.4 mg's 13.7%.
Prior to this, the efficacy gap between the two drugs had been inferred from cross-trial comparisons (SURMOUNT-1 for tirzepatide, STEP-1 for semaglutide), which are methodologically weaker than head-to-head data. SURMOUNT-5 confirmed what the cross-trial data suggested.
What the trial settled
On the primary outcome (percent body-weight reduction from baseline at 72 weeks), tirzepatide was clearly superior. The 6.5-percentage-point gap is clinically meaningful and reliable. Secondary outcomes — proportion of patients achieving ≥5%, ≥10%, ≥15%, ≥20% weight loss — also favored tirzepatide at each threshold.
Safety and tolerability were similar. Both drugs produced the expected class-typical GI side effect profile (nausea, diarrhea, constipation, vomiting), dose-dependent. Discontinuation rates for adverse events were comparable.
What the trial didn't settle
Individual response variability remains substantial. Means don't predict individuals. A patient who responds below average on tirzepatide may respond above average on semaglutide, or vice versa. Head-to-head mean data shifts the prior but does not determine which drug will work for any given person.
Cardiovascular outcomes differ between the two drugs today. Semaglutide has mature MACE reduction data (SELECT trial, 2023, NEJM). Tirzepatide's cardiovascular outcomes trial (SURPASS-CVOT) is ongoing. For patients with established cardiovascular disease, that gap matters.
Long-term (>2 year) safety and efficacy data is still accumulating for both. SURMOUNT-5 was 72 weeks; real-world durability of weight loss over years is a separate question.
Why this doesn't end the category
Retatrutide's Phase 2 data (24.2% at 48 weeks, NEJM 2023) suggests the efficacy ceiling may go higher with triple-agonism. CagriSema's REDEFINE-1 (20.4% at 68 weeks, NEJM 2025) suggests amylin + GLP-1 combinations are competitive. Both are in or near regulatory submission. Orforglipron brings an oral small-molecule option. The 'tirzepatide is the best' story is true today and likely temporary.
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