Retatrutide 2026: TRIUMPH-4 Phase 3 Shows 28.7% Weight Loss

Introduction

December 11, 2025 marked a watershed moment in obesity medicine. Eli Lilly announced that Retatrutide—the first-in-class GIP, GLP-1, and glucagon triple agonist—achieved an average weight loss of 28.7% at 68 weeks in the TRIUMPH-4 Phase 3 trial. This represents the highest weight loss ever recorded in a Phase 3 obesity trial, surpassing even the company's own dual agonist tirzepatide (Mounjaro/Zepbound), which achieved 22.5% in its pivotal studies.

For context, participants in the 12mg dose group lost an average of 71.2 pounds (32.3 kg) from a baseline weight of 248.5 pounds. Even more remarkably, 58.6% of participants achieved ≥25% total body weight loss—a threshold previously considered extraordinary in pharmacological weight management.

The TRIUMPH-4 results aren't just about numbers on a scale. This trial demonstrated that Retatrutide simultaneously addressed multiple health complications: participants experienced dramatic reductions in knee osteoarthritis pain (75.8% improvement), significant drops in blood pressure (14 mmHg systolic), and improvements in cardiovascular risk markers. More than 1 in 8 patients became completely free from knee pain by the end of the study.

With seven additional Phase 3 trials expected to read out in 2026, Retatrutide is positioning itself as potentially the most effective obesity medication ever developed. Here's everything you need to know about these groundbreaking results and what they mean for the future of weight loss treatment.

What Made TRIUMPH-4 Different From Previous Retatrutide Trials?

TRIUMPH-4 represents Eli Lilly's first successful Phase 3 trial for Retatrutide, moving beyond the promising Phase 2 results from 2023 that showed 24% weight loss. This latest trial was specifically designed to evaluate the medication in a real-world population: adults with obesity and the painful, debilitating condition of knee osteoarthritis.

Study Design and Population

The trial enrolled 445 participants across three groups in a 1:1:1 randomization:

• Retatrutide 9mg group: 148 participants
• Retatrutide 12mg group: 149 participants
• Placebo group: 148 participants

Participants had an average baseline weight of 112.7 kg (248.5 lbs) and a mean BMI of 40.4 kg/m². Notably, 84% of participants had a BMI ≥35 kg/m², placing them in the Class II or Class III obesity categories. All participants also had confirmed knee osteoarthritis based on American College of Rheumatology criteria.

The 68-week trial duration allowed researchers to assess not just rapid initial weight loss, but sustained reduction over more than a year—a critical factor in determining long-term efficacy.

Dosing Protocol

Retatrutide was administered as a once-weekly subcutaneous injection with a carefully designed titration schedule to minimize side effects:

• Weeks 1-4: 2mg
• Weeks 5-8: 4mg
• Weeks 9-12: 6mg
• Weeks 13-16: 9mg (for 9mg group) or continue to 9mg (for 12mg group)
• Week 17+: 9mg maintenance (9mg group) or 12mg maintenance (12mg group)

This gradual dose escalation over 12-16 weeks helped participants build tolerance to the medication's gastrointestinal effects while the therapeutic benefits accumulated.

The Results: Breaking Down 28.7% Weight Loss

Primary Efficacy Outcomes

Both retatrutide doses met all primary and key secondary endpoints with statistical and clinical significance:

The 12mg dose achieved the highest weight loss on record for a Phase 3 obesity trial, with more than 1 in 4 pounds lost from baseline body weight.

Weight Loss Distribution

The results weren't just impressive on average—they were consistent across the patient population:

Weight Loss Thresholds Achieved (12mg dose):

• ≥5% weight loss: 94.6% of participants
• ≥10% weight loss: 91.3% of participants
• ≥15% weight loss: 83.2% of participants
• ≥20% weight loss: 72.5% of participants
• ≥25% weight loss: 58.6% of participants

For comparison, FDA approval for obesity medications typically requires that at least 35% of patients achieve ≥5% weight loss. Retatrutide exceeded this threshold with nearly 95% of participants achieving clinically meaningful weight loss.

Pain and Physical Function Improvements

TRIUMPH-4's co-primary endpoint focused on knee osteoarthritis pain, measured by the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain subscale:

WOMAC Pain Score Reductions:

• Retatrutide 9mg: -4.5 points (75.8% reduction)
• Retatrutide 12mg: -4.4 points (73.3% reduction)
• Placebo: -2.4 points (40.3% reduction)

Participants also experienced substantial improvements in physical function, with WOMAC physical function subscale scores improving by:

• Retatrutide 9mg: -4.1 points
• Retatrutide 12mg: -4.2 points
• Placebo: -2.1 points

Perhaps most remarkably, a post-hoc analysis revealed that 14.1% of participants on 9mg and 12.0% on 12mg became completely pain-free at 68 weeks, compared to just 4.2% on placebo.

Cardiovascular and Metabolic Benefits Beyond Weight Loss

One of the most significant findings from TRIUMPH-4 was the comprehensive cardiometabolic improvements observed with retatrutide treatment.

Blood Pressure Reductions

Participants on the 12mg dose experienced a 14.0 mmHg reduction in systolic blood pressure—a clinically meaningful decrease that could translate to reduced cardiovascular events. For context, a 10 mmHg reduction in systolic blood pressure is associated with approximately 20% lower risk of major cardiovascular events.

Lipid Profile Improvements

Retatrutide produced favorable changes in multiple lipid parameters:

• Non-HDL cholesterol: Clinically meaningful reduction
• Triglycerides: Substantial decrease
• Total cholesterol: Improved levels

Inflammatory Markers

High-sensitivity C-reactive protein (hs-CRP), a key marker of systemic inflammation and cardiovascular risk, showed clinically meaningful reductions with retatrutide treatment.

These cardiovascular benefits are particularly important because obesity is a major risk factor for heart disease, stroke, and related conditions. The fact that retatrutide addresses multiple cardiovascular risk factors simultaneously—weight, blood pressure, lipids, and inflammation—suggests it could have profound impacts on long-term health outcomes.

Safety Profile: What We Learned About Side Effects

Common Adverse Events

The TRIUMPH-4 trial confirmed that retatrutide's side effect profile aligns with other incretin-based therapies, with gastrointestinal symptoms being the most common:

Incidence of GI Side Effects (12mg dose):

• Nausea: 43% (vs 9% placebo)
• Diarrhea: 33% (vs 11% placebo)
• Vomiting: 21% (vs 3% placebo)
• Constipation: Elevated vs placebo
• Decreased appetite: Common

Most gastrointestinal side effects were mild to moderate in severity and occurred primarily during the dose escalation phase. The gradual titration protocol helped minimize these effects compared to rapid dose increases.

New Safety Signal: Dysesthesia

The TRIUMPH-4 trial identified a previously unreported side effect called dysesthesia—an abnormal sensation of touch where normal sensations feel unusual, uncomfortable, or mildly painful (like tingling, numbness, or "pins and needles").

Dysesthesia Incidence:

• Retatrutide 9mg: 8.8%
• Retatrutide 12mg: 20.9%
• Placebo: 0.7%

Importantly, Eli Lilly reported that:

• Dysesthesia events were generally mild
• They infrequently led to discontinuation
• The mechanism is not yet fully understood
• This side effect was not observed in Phase 2 trials

The appearance of dysesthesia at higher doses suggests it may be dose-dependent. Researchers will be closely monitoring this side effect in the remaining TRIUMPH trials to better understand its frequency, severity, and impact on patient quality of life.

Discontinuation Rates

Treatment Discontinuation Due to Adverse Events:

• Retatrutide 9mg: 12.2%
• Retatrutide 12mg: 18.2%
• Placebo: 4.0%

Interestingly, some discontinuations were attributed to "perceived excessive weight loss"—participants felt they were losing weight too rapidly or had reached their desired weight and chose to discontinue. Eli Lilly noted that discontinuation rates were highly correlated with baseline BMI, with participants who had lower baseline BMI more likely to discontinue due to excessive weight loss.

How Retatrutide Compares to Other Weight Loss Medications

The Triple Agonist Advantage

Retatrutide's unprecedented efficacy stems from its unique mechanism as a triple hormone receptor agonist, activating three complementary pathways:

1. GLP-1 (Glucagon-Like Peptide-1): Reduces appetite, slows gastric emptying, improves insulin secretion
2. GIP (Glucose-Dependent Insulinotropic Polypeptide): Enhances GLP-1 effects, improves insulin sensitivity, may affect fat metabolism
3. Glucagon: Increases energy expenditure, promotes fat burning, may improve metabolic rate

Head-to-Head Comparison

Retatrutide's 6.2 percentage point improvement over tirzepatide and 13.7 percentage point improvement over semaglutide represents a substantial therapeutic advance. For a 250-pound person, this translates to an additional 15-34 pounds of weight loss compared to currently available options.

What This Means for Patients

The progression from single agonist (semaglutide) to dual agonist (tirzepatide) to triple agonist (retatrutide) demonstrates that targeting multiple complementary pathways can produce incrementally better outcomes. However, this also comes with:

• Higher efficacy: Substantially greater weight loss
• More side effects: Increased GI symptoms and dysesthesia
• Unknown long-term effects: Retatrutide's extended safety profile is still being evaluated

What's Next: The TRIUMPH Program Timeline

TRIUMPH-4 is just the beginning. Eli Lilly's comprehensive Phase 3 program includes seven additional trials evaluating retatrutide across multiple indications:

Expected 2026 Readouts

TRIUMPH-1 & TRIUMPH-2: Weight management trials with nested protocols for:

• Obstructive sleep apnea (OSA)
• Knee osteoarthritis
• Maintenance dosing (4mg dose)

TRIUMPH-3: Weight management in people with cardiovascular disease

Additional TRIUMPH Trials:

• Type 2 diabetes management
• Chronic low back pain
• Metabolic dysfunction-associated steatohepatolic disease (MASH, formerly NASH/NAFLD)
• Cardiovascular and renal outcomes

The TRIUMPH program has enrolled more than 5,800 participants total, making it one of the largest Phase 3 programs for an obesity medication in history.

FDA Submission Timeline

Based on the TRIUMPH-4 success and expected 2026 readouts, analysts predict:

• Late 2026: Completion of core TRIUMPH trials
• Q4 2026 or Q1 2027: New Drug Application (NDA) submission to FDA
• Mid to Late 2027: Potential FDA approval
• Early 2028: Commercial launch

Eli Lilly has not officially confirmed these timelines, but the company has indicated it will submit for regulatory approval once sufficient Phase 3 data is available.

Expert Perspectives on TRIUMPH-4

Dr. Jamy Ard, The Obesity Society Past President

"The TRIUMPH-4 topline results describing the efficacy of retatrutide in adults with obesity and knee osteoarthritis broke yet another barrier in the arms race of obesity medication therapies in development. The on-treatment efficacy estimand showed 26.6% weight loss above placebo for the highest dose of retatrutide; 58.6% of participants on that dose achieved at least 25% weight reduction.

This magnitude of weight reduction can be life-changing, and the TRIUMPH-4 trial is another example of how effectively treating obesity can improve health. No doubt, these data will likely lead to an indication in the label for the treatment of knee osteoarthritis once approved."

Kenneth Custer, PhD, Eli Lilly Executive VP

"People with obesity and knee osteoarthritis often live with pain and restricted mobility, and may eventually require total joint replacement. We are encouraged by the results of TRIUMPH-4, which highlight the powerful effect of retatrutide, a first-in-class triple agonist, on body weight, pain and physical function. With seven additional phase 3 readouts expected in 2026, we believe retatrutide could become an important option for patients with significant weight-loss needs and certain complications, including knee osteoarthritis."

Conclusion: A New Era in Obesity Treatment

The TRIUMPH-4 results represent more than just incremental progress—they signal a paradigm shift in what's possible with pharmacological obesity treatment. Achieving nearly 30% weight loss with a medication challenges previous assumptions about the upper limits of drug-induced weight reduction and brings us closer to the efficacy seen with bariatric surgery (30-40% weight loss), but without the surgical risks and recovery time.

However, several critical questions remain:

1. Will the safety profile remain acceptable as more data emerges?The dysesthesia signal in 1 in 5 patients at the highest dose warrants close monitoring. Will this side effect be tolerable for patients seeking this level of weight loss?

2. How will retatrutide perform in the remaining TRIUMPH trials?TRIUMPH-4 enrolled a specific population with knee osteoarthritis. Will results be similar in broader obesity populations, people with diabetes, or those with cardiovascular disease?

3. What will the cost be?Analysts predict retatrutide could be priced at $1,200-1,500 per month—potentially higher than current GLP-1 medications. Will insurance companies cover such an expensive treatment?

4. Can manufacturing meet demand?Shortages have plagued semaglutide and tirzepatide launches. Can Eli Lilly produce enough retatrutide to meet what will likely be enormous demand?

Despite these unknowns, TRIUMPH-4 has firmly established retatrutide as the most effective obesity medication in clinical development. For the millions of Americans and people worldwide struggling with severe obesity and related health complications, retatrutide represents hope for a truly transformative treatment option—one that could fundamentally change the trajectory of the global obesity epidemic.

The next 18 months will be critical as additional TRIUMPH data emerges and Eli Lilly navigates the regulatory approval process. One thing is certain: the obesity treatment landscape will never be the same.

Sources:

• Eli Lilly and Company. (2025, December 11). Lilly's triple agonist, retatrutide, delivered weight loss of up to an average of 71.2 lbs along with substantial relief from osteoarthritis pain in first successful Phase 3 trial [Press release]. Retrieved from https://investor.lilly.com/news-releases
• ClinicalTrials.gov Identifier: NCT05931367
• Giblin, K., et al. (2026). Retatrutide for the treatment of obesity, obstructive sleep apnea and knee osteoarthritis: Rationale and design of the TRIUMPH registrational clinical trials. Diabetes, Obesity and Metabolism, 28(1), 83-93.

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Last updated: January 15, 2026

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Retatrutide is an investigational drug not yet approved by the FDA. Always consult with a qualified healthcare provider before making decisions about weight loss treatments.