Retatrutide in 2026: What Phase 2 and Phase 3 Research Actually Shows

Evidence status: Retatrutide remains investigational. This article reports study designs and results; it does not provide a dosing recommendation or instructions for self-use.

What retatrutide is

Retatrutide (LY3437943) is an investigational peptide that activates three hormone receptors: GIP, GLP-1, and glucagon. That triple-receptor profile is the reason it has attracted substantial interest in obesity and metabolic-disease research.

What the peer-reviewed Phase 2 obesity trial found

The pivotal Phase 2 obesity study was a randomized, double-blind, placebo-controlled trial involving 338 adults with obesity, or overweight plus at least one weight-related condition. The study followed participants for 48 weeks.

At week 48, the reported mean change in body weight was −8.7% in the 1 mg group, −17.1% in the combined 4 mg groups, −22.8% in the combined 8 mg groups, and −24.2% in the 12 mg group, compared with −2.1% with placebo. The most common adverse events were gastrointestinal. The investigators also reported a dose-dependent increase in heart rate that peaked around week 24 and then declined.

Study protocol note

The trial tested weekly protocol doses of 1 mg, 4 mg, 8 mg, and 12 mg, with different starting-dose strategies in some arms. Those numbers describe the experimental design of the cited trial only. They are not a recommendation, titration plan, or guide for use outside a clinical study.

What the liver-fat substudy added

A randomized Phase 2a substudy evaluated 98 participants from the obesity trial who also had metabolic dysfunction-associated steatotic liver disease (MASLD) with liver fat of at least 10% by MRI-PDFF.

At 24 weeks, mean relative liver-fat changes were approximately −42.9%, −57.0%, −81.4%, and −82.4% across the 1 mg, 4 mg, 8 mg, and 12 mg trial groups, respectively, versus +0.3% with placebo. The study also reported reductions in visceral and subcutaneous abdominal fat.

Those findings are notable, but the limitations matter: the substudy was small, did not use liver biopsy as the primary assessment, excluded people with type 2 diabetes, and had substantially fewer MRI measurements available at week 48. The authors characterized the results as hypothesis-generating rather than definitive.

What changed in 2026: Phase 3 topline results

In 2026, Eli Lilly announced positive topline results from multiple Phase 3 TRIUMPH trials. In TRIUMPH-1, the sponsor reported average weight change at 80 weeks of −19.0% with 4 mg, −25.9% with 9 mg, and −28.3% with 12 mg, compared with −2.2% with placebo.

Lilly later reported topline results from TRIUMPH-2 in adults with obesity or overweight and type 2 diabetes, and TRIUMPH-3 in adults with severe obesity and established cardiovascular disease. The sponsor reported weight reductions up to 20.8% and 22.6%, respectively, at 80 weeks.

These 2026 Phase 3 figures are sponsor-reported topline data. Detailed peer-reviewed publications are still the stronger source for evaluating study design, subgroup behavior, uncommon adverse events, and the complete statistical analysis.

Safety signals worth watching

In the peer-reviewed Phase 2 obesity study, gastrointestinal events such as nausea, diarrhea, vomiting, and constipation were the most common adverse events. Treatment discontinuation due to adverse events occurred more often in retatrutide groups than placebo, particularly at higher study doses. A small number of serious events occurred, and the trial was not large enough or long enough to establish the full long-term safety profile.

That is one reason Phase 3 and post-approval data matter: larger populations and longer follow-up are better able to identify uncommon or delayed risks.

Regulatory status

As of September 2026, retatrutide is not approved by the U.S. Food and Drug Administration or another regulatory agency. Lilly has stated that it plans to submit a Biologics License Application to FDA in 2027. Until a regulator completes its review, efficacy findings from clinical trials should not be confused with marketing approval.

Bottom line

The strongest published evidence currently shows substantial weight loss in a randomized Phase 2 trial and substantial reductions in liver fat in a smaller MASLD substudy. Phase 3 topline results reported by the sponsor in 2026 are larger and encouraging, but full peer-reviewed reports remain important for judging the complete benefit-risk picture.