Adults living with obesity and painful knee osteoarthritis experienced substantial weight reduction and major relief from joint pain after taking the experimental injectable medication retatrutide, according to findings from two Phase 3 clinical trials published November 8 in the New England Journal of Medicine and presented at the American College of Rheumatology annual meeting in Orlando, Florida.

Led by researchers at Northwestern University and developed by pharmaceutical manufacturer Eli Lilly and Company, the two studies—TRIUMPH-4 and TRIUMPH-1-OA—evaluated more than 1,000 participants combined. In a pooled analysis of the trials reported by Northwestern University, approximately two-thirds of patients receiving retatrutide achieved at least a 50% drop in knee pain, compared with 38% of participants who received a placebo. Those on the highest dose of the weekly injection lost roughly one-quarter of their total body weight on average.

For the estimated 15 million people in the United States—roughly 5% of the population—who suffer from painful knee osteoarthritis, daily physical movements such as walking, climbing stairs, and rising from a chair can be severely restricted.

While exercise and strength training are foundational for joint health and mobility, existing joint pain often creates a barrier to physical activity.

How the Triple-Hormone Agonist Works

Unlike currently approved weight-loss medications such as semaglutide or tirzepatide, which target one or two hormone receptors and typically yield a 10% to 20% reduction in body weight, retatrutide is an investigational once-weekly injection that activates receptors for three distinct hormones: GIP, GLP-1, and glucagon. Across the study groups, this triple-receptor mechanism produced total body weight reductions between 20% and 25% at the highest doses, alongside improvements in blood pressure, lipid levels, and systemic inflammation.

Dr. Thomas Schnitzer, professor of physical medicine and rehabilitation, anesthesiology, and medicine (rheumatology) at Northwestern University Feinberg School of Medicine and corresponding author of the research, noted that reducing the physical load on the lower body directly translates to improved mobility.

"The findings are really saying for people who are overweight and have OA, if you lose a significant amount of weight with this treatment, there is a good chance that you're going to get a meaningful reduction in your knee pain," Schnitzer stated in Northwestern University's release.

"A major factor is that you're loading your knees less and therefore, there's less pain and it allows you to increase your function."

Breaking Down the Phase 3 Trial Data

While the pooled analysis reported by Northwestern University focused on the proportion of participants cutting their pain by at least half, data released by Eli Lilly and Company highlighted specific trial durations and average score changes on the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain scale.

In the 68-week TRIUMPH-4 trial—presented in Orlando by study co-author Philip Conaghan, professor of musculoskeletal medicine at the University of Leeds—participants taking the highest dose of retatrutide recorded an average 4.5-point drop in WOMAC pain scores from a baseline of 6.0, representing a 75.8% reduction. By the conclusion of the 68-week study, more than one in eight treated patients reported having no knee pain at all.

In the longer 80-week TRIUMPH-1 trial, which enrolled adults with obesity but without diabetes across 131 sites in 11 countries between July 2023 and April 2026, patients received weekly retatrutide doses of 4 mg, 9 mg, or 12 mg, or a placebo, paired with lifestyle counseling and no mandated calorie restriction.

According to Endocrinology Advisor, average body weight reductions at 80 weeks reached 17.6% for the 4 mg group, 23.7% for the 9 mg group, and 25.0% for the 12 mg group, compared with 3.9% in the placebo group. Eli Lilly reported that the highest-dose group in TRIUMPH-1 saw up to a 4.3-point reduction in WOMAC pain from a baseline of 6.0 (a 73.1% drop), with more than one in six treated participants reporting complete resolution of knee pain by the end of the trial.

Implications for Physical Mobility and Exercise

In sports medicine and physical rehabilitation, clinical guidelines consistently recommend losing 5% to 10% of body weight through nutrition and exercise to ease osteoarthritis symptoms and preserve joint mechanics. However, as Schnitzer explained, achieving more than a 10% weight reduction through lifestyle modifications alone is notoriously difficult, especially when chronic joint discomfort prevents patients from engaging in regular cardiovascular or resistance training.

Current non-surgical management for knee osteoarthritis largely relies on physical therapy, topical treatments, over-the-counter anti-inflammatory medications like ibuprofen or diclofenac (Voltaren), opioids, and corticosteroid injections, the latter of which offer only short-term symptom relief.

By substantially lowering body mass and systemic inflammation, emerging metabolic therapies could help break the cycle of pain and sedentarism, allowing patients to participate more comfortably in physical therapy and strength-building routines that support long-term joint stability.

Study Limitations and Open Clinical Questions

Despite the substantial symptom improvements documented in both trials, clinicians and researchers emphasize several important limitations. Retatrutide remains an investigational medicine and has not yet been approved by the U.S. Food and Drug Administration for either weight management or osteoarthritis treatment. Furthermore, while an earlier Phase 3 trial (STEP 9) examined semaglutide in adults with obesity and knee osteoarthritis, the recent retatrutide studies did not include an active comparator drug, meaning direct head-to-head conclusions between different medications or established osteoarthritis therapies cannot be drawn.

Coverage by Endocrinology Advisor also noted methodological constraints in TRIUMPH-1, including higher dropout rates in the placebo group, limited representation of patients with a body mass index (BMI) below 30, and the fact that the trial was not statistically powered to evaluate long-term cardiovascular safety.

Does the pain relief come solely from weight loss, or does the drug have independent joint effects?

The trial results demonstrate major pain relief alongside substantial weight reduction and lower systemic inflammation, but the studies do not isolate how much of the symptom improvement stems strictly from reduced mechanical load on the knee versus potential direct anti-inflammatory or metabolic effects of the medication itself.

Can retatrutide prevent or delay knee replacement surgery?

While the trials show significant symptom relief for many participants over 68 to 80 weeks, the available data do not establish whether the medication alters structural cartilage degeneration or reduces the long-term need for total knee replacement surgery.

Would these results apply to patients with knee osteoarthritis who do not have obesity?

Because the clinical trials specifically enrolled adults who were overweight or living with obesity, the findings cannot be generalized to individuals with normal body weight who suffer from painful knee osteoarthritis caused by prior athletic injuries, genetics, or aging.

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