Retatrutide and Osteoarthritis: TRIUMPH-4 Results

Overview

Knee osteoarthritis is one of the most prevalent and debilitating complications of obesity. Excess body weight places chronic mechanical stress on weight-bearing joints, accelerates cartilage degradation, and amplifies systemic inflammation that contributes to joint pain and functional limitation. For decades, weight loss has been recognized as a first-line intervention for knee osteoarthritis, but the magnitude of weight loss achievable through lifestyle modification alone is often insufficient to produce substantial joint symptom relief.

TRIUMPH-4, the first Phase 3 trial to report results for retatrutide, was designed specifically to evaluate the drug in adults with obesity and knee osteoarthritis. The results, announced by Eli Lilly in December 2025, demonstrated both record-setting weight loss and clinically significant improvements in knee pain and function.

TRIUMPH-4: Trial Design

TRIUMPH-4 was a randomized, double-blind, placebo-controlled Phase 3 trial with the following parameters:

• Participants: 445 adults randomized 1:1:1
• Population: Adults with obesity or overweight and knee osteoarthritis; no type 2 diabetes
• Baseline BMI: 40.4 kg/m2 (mean); 84% of participants had BMI of 35 or above
• Duration: 68 weeks
• Doses: Retatrutide 9 mg and 12 mg (with dose escalation) vs. placebo
• Co-primary endpoints: Percentage change in body weight and change in WOMAC pain score from baseline
• Source: Eli Lilly press release, December 11, 2025. Full data expected to be submitted for peer-reviewed publication; TRIUMPH program design described by Giblin et al. in Diabetes, Obesity and Metabolism (2026).

Weight Loss Results

The 28.7% mean weight loss at the 12 mg dose is the largest mean weight reduction reported for any pharmacological agent in a Phase 3 clinical trial. At a mean baseline body weight of approximately 112 kg, this translates to an average loss of 71.2 lbs (32.3 kg).

Weight Loss Thresholds

The proportion of participants achieving clinically meaningful weight loss thresholds demonstrates the consistency of response:

Nearly 60% of participants at the 12 mg dose lost at least a quarter of their body weight, and nearly 40% lost 30% or more. These magnitudes of weight reduction approach those typically associated with bariatric surgery.

Knee Pain and Function Outcomes

WOMAC Pain Scores

The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) is the standard validated instrument for assessing osteoarthritis symptoms. WOMAC pain scores range from 0 (no pain) to higher values indicating more severe pain.

The reductions in pain scores were clinically and statistically significant. A 75.8% reduction in pain scores represents a substantial improvement in daily function and quality of life.

Complete Pain Freedom

Approximately one in eight participants treated with retatrutide achieved complete freedom from knee pain by the end of the trial.

Pain Reduction Thresholds

Nearly three-quarters of participants at the 9 mg dose achieved 70% or greater pain reduction, indicating that the vast majority of treated individuals experienced substantial symptomatic relief.

WOMAC Physical Function

Functional improvements paralleled the pain reductions, with treated participants demonstrating substantially better physical function at 68 weeks. This is consistent with the hypothesis that weight loss reduces mechanical load on the knee joint and reduces obesity-associated inflammation, leading to both symptomatic and functional improvement.

Cardiovascular Biomarkers

TRIUMPH-4 also reported improvements in cardiometabolic parameters:

• Systolic blood pressure: Reduced by up to 14.0 mmHg at the 12 mg dose
• Non-HDL cholesterol, triglycerides, and hsCRP: Reduced across active treatment groups

These findings are consistent with the cardiometabolic benefits reported for retatrutide in Phase 2 and are clinically relevant given that obesity and osteoarthritis frequently co-occur with hypertension, dyslipidemia, and systemic inflammation.

Safety Profile in TRIUMPH-4

The adverse event profile in TRIUMPH-4 was consistent with the GLP-1 receptor agonist class, though some findings warranted particular attention.

Gastrointestinal Adverse Events

Dysesthesia

Dysesthesia — described as altered sense of touch including tingling, burning, numbness, or heightened skin sensitivity — was reported in 8.8% (9 mg) and 20.9% (12 mg) of participants, compared with 0.7% on placebo. This adverse event was not observed in Phase 2 trials and represents a novel safety signal for retatrutide. It was generally described as mild and self-limiting.

Discontinuation Rates

Discontinuation rates were higher in participants with lower baseline BMI, attributed in part to perceived excessive weight loss. In the subgroup with BMI of 35 or above (84% of the trial population), discontinuation rates were lower and comparable to those observed with other injectable incretin therapies. Eli Lilly has noted that this BMI-dependent pattern may influence prescribing guidance.

Clinical Significance

Weight Loss and Joint Health

The relationship between weight loss and osteoarthritis symptom improvement is well established. Each kilogram of weight lost reduces the mechanical load on the knee by approximately 4 kg during walking. At 32.3 kg of mean weight loss (12 mg group), this translates to a substantial reduction in joint stress.

However, weight loss alone may not fully explain the pain reductions observed. The placebo group lost only 2.1% of body weight yet experienced a 40.3% reduction in WOMAC pain scores — likely reflecting the placebo effect, natural history, and the impact of trial participation. The incremental benefit of retatrutide treatment (approximately 35 additional percentage points of pain reduction beyond placebo) is attributable to the combined effects of substantial weight loss, reduced systemic inflammation, and potentially direct metabolic effects of triple receptor agonism.

Comparison with Other Obesity Therapies in OA

No head-to-head trials have compared retatrutide against other obesity therapies in an osteoarthritis population. In cross-trial comparisons, retatrutide’s WOMAC pain reduction of 75.8% numerically exceeds the approximately 42% reduction reported for semaglutide in osteoarthritis trials, though differences in trial design, populations, and duration preclude direct comparison. The substantially greater weight loss achieved with retatrutide (28.7% vs. approximately 15%) is the most likely explanation for any difference in pain outcomes.

Regulatory Implications

Eli Lilly has stated plans to include osteoarthritis of the knee among the initial proposed indications when submitting the TRIUMPH program data to the FDA in late 2026. If approved, retatrutide would be one of few pharmacological agents with a specific indication for obesity-related knee osteoarthritis.

Frequently Asked Questions

How much knee pain improvement can be expected with retatrutide?

In TRIUMPH-4, WOMAC pain scores improved by 74-76% on average at the 9 mg and 12 mg doses over 68 weeks. Approximately 12-14% of participants achieved complete freedom from knee pain, and roughly 68-73% achieved 70% or greater pain reduction. These are mean values; individual responses varied. These results are from a press release (Eli Lilly, December 2025); full published data are pending.

Does retatrutide work directly on joints, or only through weight loss?

The clinical data do not separate the contributions of weight loss, reduced inflammation, and any direct effects on joint tissue. The primary mechanism is almost certainly weight loss and the resulting reduction in mechanical joint loading. Additionally, the reductions in systemic inflammatory markers (hsCRP) observed in TRIUMPH-4 may contribute to joint symptom improvement. There is no evidence of a direct effect of retatrutide on cartilage or joint tissue.

Who was eligible for the TRIUMPH-4 trial?

Participants were adults with obesity or overweight (baseline BMI averaged 40.4 kg/m2; 84% had BMI of 35 or above) and diagnosed knee osteoarthritis. Individuals with type 2 diabetes were excluded. The population was relatively homogeneous in terms of obesity severity, which should be considered when extrapolating results to individuals with lower BMI.

What about the dysesthesia side effect?

Dysesthesia (altered skin sensation) was reported in up to 20.9% of participants at the 12 mg dose, a rate not observed in Phase 2 trials. It was generally described as mild and self-limiting and did not lead to significant treatment discontinuations. This is a novel safety signal that Eli Lilly has stated warrants monitoring in subsequent TRIUMPH readouts. The mechanism is not established.

Will retatrutide be approved specifically for osteoarthritis?

Eli Lilly has stated plans to submit the TRIUMPH program data to the FDA in late 2026, with osteoarthritis of the knee listed among the proposed initial indications alongside obesity/overweight and obstructive sleep apnea. The earliest FDA decision would be mid-2027. Approval is not guaranteed and will depend on the totality of safety and efficacy data across the TRIUMPH program.

How does the weight loss in TRIUMPH-4 compare to Phase 2?

TRIUMPH-4 reported -28.7% weight loss at 12 mg over 68 weeks, exceeding the -24.2% observed at the same dose over 48 weeks in the Phase 2 obesity trial (Jastreboff et al., NEJM 2023). The greater weight loss reflects the longer treatment duration and confirms that weight loss continues beyond 48 weeks. The Phase 2 weight loss trajectories had not plateaued at 48 weeks, and the Phase 3 data demonstrate that additional weight reduction is achieved with continued treatment.

Summary

TRIUMPH-4 demonstrated that retatrutide produces both record-setting weight loss (-28.7% at 12 mg, 68 weeks) and clinically significant improvements in knee osteoarthritis symptoms (-75.8% WOMAC pain reduction at 9 mg) in adults with obesity and knee osteoarthritis. Approximately one in eight treated participants achieved complete freedom from knee pain. Cardiovascular risk markers also improved, including a 14.0 mmHg reduction in systolic blood pressure. The safety profile included notable rates of gastrointestinal adverse events and a dysesthesia signal affecting up to 20.9% at the highest dose. These are the first Phase 3 results for retatrutide, reported via Eli Lilly press release in December 2025. Full peer-reviewed publication is pending.