Retatrutide: The Triple Agonist That Lost Participants 71 lbs in Phase 3

What is Retatrutide?

Retatrutide (LY3437943) is a first-in-class investigational peptide developed by Eli Lilly that activates three hormone receptors simultaneously: GIP (glucose-dependent insulinotropic polypeptide), GLP-1 (glucagon-like peptide-1), and glucagon. This triple agonist mechanism is what separates it from existing treatments like semaglutide (GLP-1 only) and tirzepatide (GIP/GLP-1 dual agonist).

The addition of the glucagon receptor is the key differentiator. Glucagon activation increases energy expenditure and promotes fat oxidation — essentially increasing how many calories the body burns — on top of the appetite suppression provided by GLP-1 and the insulinotropic effects of GIP.

Phase 3 TRIUMPH-4 Trial Results (December 2025)

In December 2025, Eli Lilly announced topline results from the TRIUMPH-4 Phase 3 trial, which evaluated retatrutide in adults with obesity or overweight who also had knee osteoarthritis. The results were significant:

• Weight loss: Participants on the highest dose (12 mg) who completed the 68-week study lost an average of 28.7% of their body weight — approximately 71.2 lbs (32.3 kg)
• Osteoarthritis pain: Retatrutide delivered substantial relief from knee OA pain, marking the first successful Phase 3 trial to demonstrate both weight loss and joint pain improvement
• Cardiovascular markers: Significant reductions in non-HDL cholesterol, triglycerides, and high-sensitivity C-reactive protein (hsCRP)
• Blood pressure: The 12 mg dose lowered systolic blood pressure by an average of 14.0 mmHg

How Does It Compare?

To put these numbers in context against other peptides in the weight loss category:

• Semaglutide (Wegovy) — achieved approximately 15-17% weight loss in Phase 3 trials
• Tirzepatide (Zepbound) — achieved approximately 20-22% weight loss in Phase 3 trials
• Retatrutide — achieved up to 28.7% weight loss in Phase 3, a substantial leap over existing options

The addition of the glucagon receptor appears to be driving the incremental weight loss beyond what dual agonists can achieve. You can compare these peptides directly using the Peptadex compare tool.

The TRIUMPH Clinical Program

TRIUMPH is Eli Lilly's comprehensive Phase 3 program for retatrutide, consisting of eight clinical trials evaluating the compound across multiple conditions:

• TRIUMPH-1: Obesity without type 2 diabetes
• TRIUMPH-2: Type 2 diabetes
• TRIUMPH-3: Obesity with type 2 diabetes
• TRIUMPH-4: Obesity with knee osteoarthritis (results reported December 2025)
• TRIUMPH-5 through 8: Additional populations and endpoints

The remaining seven trials are expected to report results throughout 2026, with potential FDA submission following completion of the full program.

Safety and Tolerability

The most common adverse events reported in the TRIUMPH-4 trial were gastrointestinal in nature, consistent with the GLP-1 class:

• Nausea: 38.1% (9 mg) to 43.2% (12 mg) vs placebo
• Diarrhea: 34.7% to 33.1%
• Constipation: 21.8% to 25.0%
• Vomiting: 20.4% to 20.9%
• Decreased appetite: 19.0% to 18.2%

Notably, some participants discontinued the study due to excessive weight loss — an unusual finding that underscores the potency of the triple agonist mechanism. The discontinuation rate is something the FDA will likely scrutinize during the review process.

Dosing and Administration

Based on trial protocols, retatrutide is administered as a once-weekly subcutaneous injection, similar to semaglutide and tirzepatide. The titration schedule starts at a low dose and gradually increases over several weeks to the target dose of 8 mg or 12 mg weekly. You can view the full dosing details on the retatrutide peptide page.

What Comes Next

As of early 2026, retatrutide is not yet FDA approved and is only available through clinical trials. Key milestones to watch:

• 2026: Results from the remaining seven TRIUMPH trials expected throughout the year
• 2026-2027: Potential FDA submission if the full TRIUMPH program demonstrates consistent efficacy and safety
• 2027-2028: Potential FDA approval and commercial launch, depending on review timeline

The weight loss peptide landscape is evolving rapidly. With semaglutide and tirzepatide already on the market and retatrutide potentially joining them, the triple agonist approach represents the next frontier in metabolic medicine.

Key Takeaways

• Retatrutide is the first triple hormone receptor agonist (GIP/GLP-1/glucagon) to reach Phase 3
• The TRIUMPH-4 trial showed 28.7% weight loss at the highest dose — significantly more than semaglutide or tirzepatide
• The glucagon receptor activation drives additional fat burning beyond appetite suppression alone
• Cardiovascular risk markers improved significantly across the board
• GI side effects are common but consistent with the GLP-1 class
• Seven more Phase 3 trials are expected to report in 2026

This article is for educational and informational purposes only. It is not medical advice. Retatrutide is an investigational compound not yet approved by the FDA. Always consult a qualified healthcare professional before making decisions about your health.