On May 21, 2026, Eli Lilly announced topline results from TRIUMPH-1, the first pivotal Phase 3 obesity trial for retatrutide. The 12 mg dose produced 28.3% mean weight loss at 80 weeks. In participants with a starting BMI of 35 or higher who continued treatment to 104 weeks, the mean weight loss was 30.3%, an average of 85.0 lbs lost.
That is the largest weight loss ever recorded in a Phase 3 obesity trial. It matches the outcomes seen with bariatric surgery. Two successful Phase 3 readouts are now in hand.
Retatrutide (LY3437943) is a once-weekly injectable triple hormone receptor agonist. It activates three receptors simultaneously: GLP-1, GIP, and glucagon. Every approved GLP-1 therapy activates one of those receptors. Tirzepatide, the current efficacy benchmark, activates two. Retatrutide activates all three.
The glucagon receptor component is the mechanistic reason for the larger weight loss. Glucagon receptor activation directly stimulates hepatic fatty acid oxidation through pathways that GLP-1 and GIP receptor agonism cannot replicate with equivalent potency. Adding glucagon to GLP-1 and GIP produces a distinct metabolic profile: more direct liver fat reduction, higher thermogenic effect, and a steeper weight loss curve that had not plateaued at 80 weeks in Phase 2.
TRIUMPH-1 enrolled adults with obesity or overweight and at least one weight-related comorbidity, without type 2 diabetes. Primary endpoint: percentage change in body weight from baseline to 80 weeks.
Results at 80 weeks by dose:
12 mg: 28.3% mean weight loss. 4 mg: 19.0% mean weight loss. Placebo: pending full publication. Discontinuation due to adverse events: 4.1% at 4 mg, 6.9% at 9 mg, 11.3% at 12 mg, versus 4.9% with placebo.
104-week extension (BMI 35 or higher, 12 mg):
30.3% mean weight loss, averaging 85.0 lbs. The adverse event profile is consistent with the GLP-1 class: gastrointestinal events dominate, mostly nausea and vomiting, mostly mild to moderate, mostly resolving during treatment. Cardiometabolic secondary endpoints were positive across the board: waist circumference, non-HDL cholesterol, triglycerides, systolic blood pressure, and high-sensitivity C-reactive protein all improved from baseline.
Direct head-to-head Phase 3 data does not yet exist. TRIUMPH-5 is an active comparator trial but has not reported. The comparison below uses matched-duration Phase 3 trials with different populations and different trial designs and should be read with that caveat in mind.
Retatrutide 12 mg (TRIUMPH-1, 80 weeks): 28.3%. Retatrutide 12 mg (TRIUMPH-1 extension, 104 weeks): 30.3%. Tirzepatide 15 mg (SURMOUNT-1, 72 weeks): 22.5%. Semaglutide 2.4 mg (STEP-1, 68 weeks): 14.9%. CagriSema 2.4/2.4 mg (REDEFINE 1, 68 weeks): 22.7%. Bariatric surgery (various, 12 to 24 months): 25 to 35%.
The 30.3% figure at 104 weeks sits at the lower end of bariatric surgery outcomes. That is a meaningful statement about pharmacological efficacy.
Two successful Phase 3 readouts (TRIUMPH-1 and TRIUMPH-4, which showed 28.7% weight loss and 75.8% knee pain reduction in osteoarthritis) put Lilly in position to file an NDA. Seven additional TRIUMPH readouts are expected in 2026, including TRIUMPH-2 in type 2 diabetes and TRIUMPH-3 in established cardiovascular disease. An NDA filing is anticipated in late 2026 to early 2027.
Full TRIUMPH-1 results will be presented at the 86th Annual American Diabetes Association Scientific Sessions. Topline press releases precede peer-reviewed publication by months, so the complete dataset is not yet available.
Topline results cover the primary weight loss endpoint. The full safety dataset, the cardiovascular outcomes data, the subgroup analyses by comorbidity, and the dose-response detail for the 9 mg arm are all pending peer-reviewed publication.
The discontinuation rate of 11.3% at the 12 mg dose is higher than the 4.9% placebo rate. That is a real signal and will matter for real-world tolerability at the highest dose. The 4 mg arm at 19.0% with a 4.1% discontinuation rate may be the clinically preferred dose for many patients when tolerability is weighted alongside efficacy.
Retatrutide is not approved anywhere. It is an investigational compound. There is no prescription pathway and no approved formulation as of May 2026.
TRIUMPH-1 is the most important Phase 3 result in the metabolic medicine space since SURMOUNT-1. Two successful readouts. An NDA filing window opening later this year. A weight loss number that for the first time competes directly with surgery in a controlled trial.
The full retatrutide evidence profile, covering all researched applications including the TRIUMPH-4 osteoarthritis data and the complete safety picture across trials, is available to Signal Pro subscribers at thepeptidesignal.com.
Eli Lilly and Company. (2026). TRIUMPH-1 topline results press release. May 21, 2026.
Jastreboff AM, et al. (2023). New England Journal of Medicine. Phase 2 retatrutide dose-finding results.
Eli Lilly and Company. (2025). TRIUMPH-4 topline results press release. December 2025.
Every Tuesday: one landmark study, one protocol, one regulatory update. Plus new research posts as they publish. Free.
For research and educational purposes only · Not medical advice · Consult a qualified physician before any human use