Retatrutide (Eli Lilly code LY3437943) is a once-weekly triple-hormone-receptor agonist — a single molecule that activates the GIP, GLP-1, and glucagon receptors. On paper, the premise is additive: GLP-1 agonism drives satiety and slows gastric emptying, GIP co-agonism amplifies the glycemic and weight-loss signal (the basis of tirzepatide's edge over semaglutide), and glucagon receptor agonism adds energy expenditure through hepatic lipid mobilization.
The question the field has been working through since 2023 is whether that theoretical additivity translates into a durable, tolerable weight-loss magnitude beyond what dual-agonist tirzepatide already delivers. The Phase 2 data said yes in a sample of 338 patients. The TRIUMPH Phase 3 program, launched in 2023, is the confirmatory answer. As of April 2026, one of the eight TRIUMPH readouts has published top-line data — TRIUMPH-4, the obesity + knee-osteoarthritis study, reported in December 2025 — and seven more are scheduled to report across 2026.
This piece walks through what Phase 2 established, what TRIUMPH-4 added, and what is known about the remaining registrational trials.
The Phase 2 baseline: Jastreboff et al., NEJM 2023
Jastreboff and colleagues randomized 338 adults with BMI ≥30 (or ≥27 with at least one weight-related complication) to subcutaneous retatrutide 1 mg, 4 mg, 8 mg, or 12 mg once weekly — or placebo — for 48 weeks. The primary endpoint was percentage change in body weight.
At 48 weeks, the least-squares mean percentage change in body weight was –8.7% in the 1 mg group, –17.1% in the combined 4 mg group, –22.8% in the combined 8 mg group, and –24.2% in the 12 mg group, versus –2.1% on placebo. The adverse-event profile was consistent with the GLP-1 class — nausea, diarrhea, and vomiting, largely dose-dependent and concentrated during dose escalation. There was no plateau at 48 weeks, which is the detail that made the field pay attention: the 8 mg and 12 mg arms were still losing weight when the trial ended.
That result is the benchmark against which every subsequent retatrutide readout is measured. It is also why Lilly moved straight into a broad Phase 3 program rather than a single confirmatory obesity trial.
The TRIUMPH program
The TRIUMPH rationale-and-design paper (Giblin et al., Diabetes, Obesity and Metabolism, 2026) describes a registrational program enrolling more than 5,800 participants across eight Phase 3 trials, using a basket design that evaluates obesity alongside multiple obesity-related complications — obstructive sleep apnea and knee osteoarthritis among them — concurrently rather than sequentially. The headline indications are weight management, sleep apnea, and osteoarthritis. Several trials also enroll participants with type 2 diabetes.
The program is structured so that readouts cluster across 2026 rather than sequencing one obesity trial at a time. That structure is the reason a reader in mid-2026 sees a lot of near-term pipeline activity on retatrutide: Lilly is intentionally bunching the data-generation window.
TRIUMPH-4: the first Phase 3 readout (December 2025)
TRIUMPH-4 (NCT05931367) randomized 445 adults with obesity or overweight plus knee osteoarthritis to retatrutide 9 mg, retatrutide 12 mg, or placebo, in a 1:1:1 ratio. The study was double-blind, placebo-controlled, and 68 weeks in duration. Top-line results were released by Lilly on December 4, 2025.
Participants in the 12 mg arm lost an average of 28.7% of body weight at 68 weeks. WOMAC pain scores improved by up to an average of 4.5 points — roughly a 75.8% reduction from baseline — and more than one in eight participants on retatrutide reported being completely free from knee pain at the end of the trial. The adverse-event profile again clustered in GI tolerability, consistent with Phase 2 and with the broader incretin class.
One important framing point. TRIUMPH-4 was not a head-to-head against tirzepatide. The 28.7% figure is a within-trial placebo-controlled result in an obesity-plus-OA population over 68 weeks. Cross-trial comparisons against SURMOUNT-1 (tirzepatide) and STEP-1 (semaglutide) require the usual caveats about population, duration, and endpoint definition.
What is still outstanding
Seven additional TRIUMPH readouts are expected across 2026. The flagship obesity trial in the program — TRIUMPH-1 (NCT05929079) — is the one the field is watching most closely because it is the largest general-obesity registrational study and tests the 4 mg, 9 mg, and 12 mg doses against placebo. There is also a maintenance-dose arm at 4 mg designed to probe whether patients can step down from an induction dose and hold the weight loss. That maintenance question has been an open issue across the GLP-1 class since STEP-4 showed that semaglutide discontinuation is followed by substantial weight regain.
Until those readouts publish, the honest summary of retatrutide evidence as of April 2026 is: one large Phase 2 trial with clean results, one Phase 3 readout in a complicated-patient population with a specific primary endpoint (weight + WOMAC pain), and seven more Phase 3 readouts still to come. Any comparison to tirzepatide or semaglutide today is Phase 2 data projected forward — which is the right framing for readers deciding whether to track the pipeline or wait for the registrational dataset.
PeptaHub's GLP-1 pillar tracks the broader class evidence; retatrutide's individual peptide profile is updated as each Phase 3 readout publishes.