Eli Lilly's Retatrutide Achieves Up to 30% Weight Loss in Phase 3 TRIUMPH-1 Trial

May 21, 2026

Eli Lilly's experimental drug Retatrutide achieved up to 30.3% weight loss in its Phase 3 TRIUMPH-1 trial, a level comparable to bariatric surgery. The once-weekly, triple-agonist injection targets GLP-1, GIP, and glucagon receptors, outperforming existing treatments. While highly effective, the drug's highest doses caused significant gastrointestinal side effects, though a lower, better-tolerated dose still produced 19% weight loss.

TRIUMPH-1 Phase 3 results

▪The Phase 3 TRIUMPH-1 trial was a randomized, double-blind, placebo-controlled study involving 2,339 participants without diabetes.
▪A Phase 2 study showed Retatrutide reduced liver fat by approximately 86% in participants with metabolic dysfunction-associated steatotic liver disease (MASLD).

Triple-agonist receptor mechanism

▪Retatrutide activates receptors for glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and glucagon.
▪Eli Lilly's Retatrutide is an investigational, once-weekly triple hormone receptor agonist for adults with obesity or overweight.

Weight loss efficacy benchmarks

▪In a trial extension to 104 weeks, participants on the 12 mg dose achieved an average weight loss of 30.3%, or 85.0 pounds.
▪The 4 mg dose of Retatrutide resulted in an average weight loss of 19.0%, or 47.2 pounds, over 80 weeks.
▪At the highest dose (12 mg), patients lost an average of 28.3% of their body weight, or 70.3 pounds, over 80 weeks.
▪At 80 weeks, 45.3% of participants on the 12 mg dose achieved weight loss of 30% or more, a level historically associated with bariatric surgery.

Gastrointestinal side effects

▪The safety profile of Retatrutide was consistent with other incretin-based therapies, with gastrointestinal issues being the most common.
▪The discontinuation rate due to side effects was 11.3% for the 12 mg dose, compared to 4.1% for the 4 mg dose and 4.9% for placebo.
▪At the 12 mg dose, common side effects included nausea (42.4%), diarrhea (32.0%), constipation (26.1%), and vomiting (25.3%).

Competitive obesity drug landscape

▪Eli Lilly's existing obesity drug tirzepatide (Zepbound) achieved up to 22.5% weight loss in its key trial.
▪Analysts expect Eli Lilly to file for FDA approval of Retatrutide in late 2026 or early 2027.

FDA approval timeline expectations

▪The topline data from the TRIUMPH-1 trial was released on May 21, 2026, and has not yet been peer-reviewed or published.
▪A realistic approval window for Retatrutide is estimated to be late 2027 to early 2028, following a standard FDA review.