Retatrutide Phase 3 Results (2026): Everything Researchers Need to Know About the TRIUMPH Trials

Retatrutide Phase 3 TRIUMPH-4 results show up to 28.7% body weight reduction at 68 weeks. Full trial data, timeline, and what researchers need to know in 2026.

Retatrutide (LY3437943) is an investigational once-weekly triple hormone receptor agonist developed by Eli Lilly that targets the GIP, GLP-1, and glucagon receptors simultaneously. The first Phase 3 results from the TRIUMPH-4 trial, released in December 2025, reported up to 28.7% body weight reduction and significant pain score improvements in a 68-week study of 445 participants. Seven more Phase 3 readouts are expected throughout 2026.

At a Glance: Retatrutide Clinical Development Status

All data reflects published clinical trial results and Eli Lilly press releases. Retatrutide is not FDA approved and is available for research purposes only.

What Makes This Compound Different? The Triple Agonist Mechanism

Here's the thing. Most compounds in this research space target one receptor. Some target two. Retatrutide goes after three.

It's a 39-amino-acid synthetic peptide built on a GIP backbone with three non-coded amino acid residues at positions 2, 13, and 20. The position 2 modification (Aib, or aminoisobutyric acid) blocks DPP-4 cleavage, which means the molecule sticks around longer. A C20 fatty diacid conjugation enables albumin binding, giving it a half-life of roughly 6 days. That's what makes once-weekly administration possible in clinical settings.

In published in vitro assays, retatrutide binds human glucagon receptor (GCGR), GIP receptor (GIPR), and GLP-1 receptor (GLP-1R) with EC₅₀ values of 5.79 nM, 0.0643 nM, and 0.775 nM, respectively.

So why does the triple mechanism matter to researchers?

The GLP-1 receptor activation pathway is well-documented in published literature. GIP receptor agonism adds a second metabolic pathway. But the glucagon receptor component is what separates retatrutide from dual agonists in preclinical models. Published research from Cell Metabolism (2022) by Coskun et al. described how the glucagon component appears to play a role in energy expenditure and lipid metabolism in laboratory settings.

The TRIUMPH Phase 3 Program: Full Trial Breakdown

Lilly's TRIUMPH program isn't one trial. It's eight. And it's the biggest clinical development program for a triple agonist to date. The program launched in 2023 and has enrolled over 5,800 participants across multiple indications.

The basket trial design is worth noting. TRIUMPH-1 and TRIUMPH-2 use a "basket" structure where OSA and OA sub-studies are nested within the main weight management trial. This lets Lilly evaluate the compound across related conditions simultaneously without running entirely separate programs.

TRIUMPH-1 and TRIUMPH-2 also include a 4 mg maintenance group in addition to the 9 mg and 12 mg groups tested in TRIUMPH-4. That maintenance data will be interesting for researchers looking at long-term protocols.

Citi analysts have noted that TRIUMPH-1 could show over 30% weight reduction because of its longer 80-week duration. We'll see.

TRIUMPH-4 Results: The Numbers

Lilly released topline TRIUMPH-4 results on December 11, 2025. The full dataset hasn't been published in a peer-reviewed journal yet (that's expected at a future medical conference), but the topline numbers are public.

Study Design

TRIUMPH-4 (NCT05931367) was a Phase 3, 68-week, randomized, double-blind, placebo-controlled study. It enrolled 445 adults with obesity or overweight and knee osteoarthritis (no diabetes). Participants were randomized 1:1:1 to retatrutide 9 mg, retatrutide 12 mg, or placebo.

Baseline characteristics: mean body weight of 112.7 kg (248.5 lbs), mean BMI of 40.4 kg/m².

Coprimary endpoints: percent change in body weight and change in WOMAC pain subscale score at week 68.

Weight Outcomes (Efficacy Estimand)

The placebo-adjusted weight reduction for the 12 mg group was approximately 26.6%. Nearly half the participants on 12 mg achieved 25% or greater weight reduction. And 58.6% of participants on the highest level achieved at least 25% weight reduction.

WOMAC Pain Outcomes

Look at those pain numbers. A 75.8% reduction versus 40.3% for placebo. And in a post-hoc analysis, 14.1% of the 9 mg group and 12.0% of the 12 mg group reported being completely free of knee pain at week 68. Only 4.2% of placebo participants hit that mark.

Physical function scores followed a similar pattern. WOMAC physical function subscale reductions exceeded 70% for both retatrutide groups.

Cardiometabolic Markers

Lilly reported clinically meaningful improvements in non-HDL cholesterol, high-sensitivity C-reactive protein, and triglycerides. Systolic blood pressure dropped by 14.0 mmHg at the 12 mg level. Those are notable secondary findings.

Safety Profile: What Researchers Should Know

Now for the part that doesn't make the headlines. Every compound has a safety profile, and researchers need the full picture.

GI Events

The most commonly reported adverse events were gastrointestinal, consistent with other incretin-class compounds in published literature. Nausea rates hit 43%, vomiting 21%, and diarrhea 33% across the retatrutide groups. These were generally mild to moderate and most common during the escalation period.

Discontinuation Rates

Overall treatment discontinuation rates were similar across groups. But discontinuation due to adverse events was higher with retatrutide: 12.2% (9 mg) and 18.2% (12 mg) versus 4.0% with placebo. Lilly noted these rates were highly correlated with baseline BMI and some were due to "perceived excessive weight loss." For participants with baseline BMI of 35 or above, the adverse event discontinuation rates dropped to 8.8% and 12.1% for the 9 mg and 12 mg groups.

Phase 2 to Phase 3: How the Data Compares

The Phase 2 trial (published in The New England Journal of Medicine, August 2023, Jastreboff et al.) enrolled 338 adults and tested multiple escalation regimens across 48 weeks. At the 12 mg level, the Phase 2 results showed 24.2% body weight reduction at 48 weeks.

TRIUMPH-4 ran 20 weeks longer (68 weeks total) and showed 28.7% at the 12 mg level. That's a meaningful jump, and it tracks with what researchers expected from the extended treatment duration.

But can we directly compare these numbers? Not exactly. The Phase 2 population had no specific comorbidity requirement beyond BMI criteria. TRIUMPH-4 specifically enrolled participants with knee osteoarthritis. Different populations, different baseline characteristics, different trial durations.

What's consistent: the dose-response relationship held up. Higher levels produced greater weight reduction in both trials. And the GI side effect profile looked similar.

What's new: the dysesthesia signal, which appeared only in Phase 3.

What's Coming in 2026

2026 is going to be a big year for retatrutide data. Seven Phase 3 readouts are expected, and each one adds a piece to the puzzle.

The ones to watch most closely:

As for FDA filing, here's the reality: Eli Lilly has not announced an official NDA submission timeline. Industry analysts project a late 2026 filing with potential approval in late 2026 to early 2027. But those are projections, not confirmed dates. The remaining TRIUMPH data, particularly the safety profile across larger populations, will determine that timeline.

Why This Matters for the Research Community

Retatrutide represents a new class of research compound. Triple agonism at GIP, GLP-1, and glucagon receptors is a mechanism that wasn't available for laboratory study until recently. The published data from both Phase 2 and Phase 3 trials gives researchers a growing body of literature to build on.

For labs working with receptor agonists, the EC₅₀ values (GCGR: 5.79 nM, GIPR: 0.0643 nM, GLP-1R: 0.775 nM) provide clear benchmarks for in vitro comparison studies. The compound's 39-amino-acid structure with three non-coded residues at positions 2, 13, and 20 makes it a useful reference for peptide engineering research.

And for researchers sourcing this compound for laboratory work, purity and verification matter more than ever with a compound this active. Third-party testing through accredited labs ensures you're working with what you think you're working with.

Timeline: Retatrutide Clinical Development

Frequently Asked Questions

For research use only. Not for human consumption. WhyNot Labs does not make health claims about any compound. All information in this article is sourced from published peer-reviewed research and publicly available clinical trial data.

Written by Ash, Founder of WhyNot Labs. All WhyNot Labs products are independently tested by Vanguard Laboratory with full Certificates of Analysis published for every batch.