Retatrutide TRIUMPH-1: 28% Weight Loss in Phase 3
Lilly's TRIUMPH-1 trial showed retatrutide 12 mg produced 28.3% mean weight loss at 80 weeks in 2,339 adults with obesity — exceeding any approved drug.
May 26, 2026 · 3 min read
Eli Lilly announced topline Phase 3 results for retatrutide on May 21, 2026, reporting that its triple-agonist peptide produced up to 28.3% mean weight loss at 80 weeks in adults with obesity but without diabetes — the highest efficacy number from any large placebo-controlled obesity trial to date.
What happened
The TRIUMPH-1 trial enrolled 2,339 adults with a BMI of 30 or above, or 27 or above with at least one weight-related health condition, who did not have type 2 diabetes. Participants received weekly subcutaneous injections of retatrutide (at 4 mg, 9 mg, or 12 mg) or placebo for 80 weeks.
All three active doses met the trial's primary and key secondary endpoints. At 80 weeks, mean weight loss by dose was:
| Dose | Mean weight loss | Mean pounds lost |
|---|---|---|
| 4 mg | 19.0% | 47.2 lb |
| 9 mg | 25.9% | 64.4 lb |
| 12 mg | 28.3% | 70.3 lb |
| Placebo | 2.2% | — |
Among participants on the 12 mg dose, 45.3% achieved 30% or greater weight loss — a threshold historically associated with bariatric surgery outcomes — and 65.3% reached a BMI below 30 by week 80. In a pre-specified extension of the trial for participants with baseline BMI of 35 or above who continued on retatrutide 12 mg to 104 weeks, mean weight loss reached 30.3%, representing an average of 85 pounds lost.
Why it matters
TRIUMPH-1 is the pivotal registration trial for retatrutide in obesity. These numbers will form the basis of Lilly's FDA submission, which the company has indicated it plans to file in 2026. Approval would give physicians a drug with meaningfully higher efficacy than either semaglutide (roughly 15% weight loss in STEP-1) or tirzepatide (up to 22.5% in SURMOUNT-1).
The 104-week extension data showing 30.3% loss is particularly notable for the regulatory argument: Lilly can demonstrate that weight loss continues to accumulate past the 80-week primary endpoint, which may support labeling that reflects the drug's full potential rather than just the snapshot at the primary timepoint.
Retatrutide targets three receptors — GLP-1, GIP, and glucagon — compared with tirzepatide's two (GLP-1 and GIP). The glucagon component is thought to increase energy expenditure, which may explain why weight loss exceeds what the dual-agonist achieved in SURMOUNT-1. For more on how these drugs differ mechanically, see Where retatrutide fits in the GLP-1 drug tree.
What to watch
Lilly still needs to publish peer-reviewed trial data; topline announcements contain limited detail on safety, demographics, and secondary endpoints. The full dataset is expected to appear in a major journal or at a medical conference later in 2026. Safety signals — particularly cardiovascular outcomes, pancreatitis, and thyroid — will be scrutinized in the full publication.
The separate TRIUMPH-4 cardiovascular outcomes trial is still ongoing, with completion expected in 2027. Whether retatrutide produces the MACE benefit semaglutide demonstrated in SELECT will be a key factor in payer coverage decisions.
Sources
Sources