Eli Lilly (NYSE: LLY) reported that retatrutide, its investigational GIP, GLP-1, and glucagon triple hormone receptor agonist, produced a mean body weight reduction of 28.3% at 80 weeks in the pivotal Phase III TRIUMPH-1 trial. A total 45.3% of participants on the highest dose achieved at least 30% weight loss — a threshold characterized by Lilly as associated with bariatric surgery outcomes.
TRIUMPH-1 is a Phase III, 80-week, randomized, double-blind, placebo-controlled trial that enrolled 2,339 adults with obesity or overweight and at least one weight-related comorbidity, without type 2 diabetes. Participants received retatrutide at 4 mg, 9 mg, or 12 mg doses or placebo, with a mean baseline weight of 112.7 kg and BMI of 40.0 kg/m².
All three doses met the primary and key secondary endpoints at 80 weeks. Participants on 12 mg lost a mean of 70.3 lbs (28.3%), those on 9 mg lost 64.4 lbs (25.9%), and those on the 4 mg dose — reached with a single escalation step — lost 47.2 lbs (19.0%), compared with 5.5 lbs (2.2%) for placebo. On the treatment-regimen estimand, which accounts for adherence regardless of protocol, the 12 mg arm produced a mean 25.0% reduction.
Among participants on 12 mg, 62.5% achieved at least 25% body weight reduction and 27.2% achieved at least 35% reduction. Waist circumference declined by a mean of 24.1 cm from a baseline of 118.3 cm in the 12 mg arm, versus 3.6 cm with placebo. Retatrutide also produced improvements from baseline in non-HDL cholesterol, triglycerides, systolic blood pressure, and high-sensitivity C-reactive protein, though specific numerical data for those endpoints were not disclosed in the topline release.
In a pre-specified blinded extension enrolling 532 participants with a baseline BMI of at least 35 who completed the main 80-week study, those continuing on retatrutide 12 mg to a maximum tolerated dose reached a mean weight loss of 85.0 lbs (30.3%) at 104 weeks. Participants who had been on placebo during the main trial and crossed over to retatrutide in the extension lost a mean of 49.9 lbs (19.2%) over the same period.
The adverse event profile was consistent with other incretin-based therapies. The most common events in the 12 mg arm were nausea (42.4% vs. 14.8% placebo), diarrhea (32.0% vs. 13.5%), constipation (26.1% vs. 10.9%), and vomiting (25.3% vs. 4.8%). Dysesthesia occurred in 12.5% of participants on 12 mg compared with 0.9% on placebo; events were generally mild to moderate, the majority resolved during treatment, and most participants continued on retatrutide. Discontinuation due to adverse events was 11.3% for the 12 mg arm, 6.9% for 9 mg, and 4.1% for 4 mg, versus 4.9% for placebo. The notably lower discontinuation rate in the 4 mg arm relative to placebo reflects a tolerability profile that Lilly has positioned as a differentiating feature for patients earlier in their treatment journey.
Competitive positioning
Retatrutide's mechanism distinguishes it from currently approved options. As a triple agonist activating GLP-1, GIP, and glucagon receptors simultaneously, it adds glucagon receptor engagement — theorized to increase energy expenditure — on top of the dual incretin signaling that defines Lilly's own Zepbound (tirzepatide), its approved GIP/GLP-1 dual agonist. In SURMOUNT-1, tirzepatide produced a mean weight reduction of approximately 20–22% at 72 weeks in a comparable non-diabetic obesity population, establishing the current efficacy benchmark among approved agents. Novo Nordisk's Wegovy (semaglutide), a GLP-1 receptor monoagonist, demonstrated approximately 15% mean weight reduction in the STEP-1 trial. Cross-trial comparisons are limited by differences in patient populations, baseline BMI, trial duration, and estimand definitions, and direct head-to-head data between retatrutide and either tirzepatide or semaglutide are not available from the TRIUMPH-1 readout.