Retatrutide and knee osteoarthritis: what TRIUMPH-4 found

What TRIUMPH-4 tested

Eli Lilly's TRIUMPH program is running five Phase 3 trials on retatrutide, each aimed at a different population: obesity alone, obesity with diabetes, obesity with cardiovascular disease, type 2 diabetes on its own, and now, with TRIUMPH-4, obesity combined with knee osteoarthritis. It is the first purpose-built Phase 3 trial for any GLP-1-class drug with knee osteoarthritis as the primary population, not a side observation in a weight-loss study.

The trial enrolled 445 adults with a mean body weight of 112.7 kg and an average BMI of 40.4. Everyone had radiographically confirmed knee osteoarthritis and a baseline WOMAC pain subscale score around 6 out of 10. Participants were split 1:1:1 into three arms: weekly retatrutide at 9mg, weekly retatrutide at 12mg, or placebo, over 68 weeks. People with type 2 diabetes were excluded, isolating the osteoarthritis and weight effects from diabetes-related confounders.

The weight loss numbers

On the efficacy estimand, the 9mg group lost 26.4% of body weight and the 12mg group lost 28.7%, or about 71 pounds on average. Placebo lost 2.1%. Those numbers track closely with what retatrutide has shown in its other Phase 3 trials, and they are still ahead of tirzepatide's roughly 22.5% and semaglutide's roughly 14.9% in head-to-head obesity trials.

Nearly 59% of people on the 12mg dose lost at least a quarter of their body weight. Almost a quarter lost more than 35%. Those are numbers that put a meaningful dent in joint load: a 220-pound person losing 28.7% comes down to roughly 157 pounds, a difference that matters a great deal to a knee bearing that weight thousands of times a day.

The pain data, specifically

The WOMAC pain subscale runs from 0 to 10, and participants started around 6. By week 68, the 9mg group dropped by 4.5 points (a 75.8% reduction) and the 12mg group dropped by 4.4 points (74.3%). Placebo dropped by 2.4 points, or 40.3%, which is itself notable, placebo response in pain trials tends to run high, but retatrutide still nearly doubled it.

More strikingly, 73% of the 9mg group and 67.7% of the 12mg group reached a 70% or greater reduction in pain score, against 26.2% on placebo. And 14.1% of the 9mg group and 12% of the 12mg group reported their knee pain had completely resolved by the end of the trial, versus 4.2% on placebo. For a condition that currently has no disease-modifying drug treatment, only pain management and eventual joint replacement, a one-in-eight full resolution rate from a weekly injection is a genuinely new data point.

Weight loss, inflammation, or both

The obvious explanation for less knee pain is less weight on the knee. Each pound of body weight adds roughly four pounds of force across the knee joint during walking, so a 70-pound weight loss removes something like 280 pounds of cumulative load per step. That alone would explain a large share of the pain improvement.

But mechanical load is probably not the whole story. GLP-1 receptors are expressed on immune cells and synovial tissue within joints, and a 2026 systematic review of GLP-1 receptor agonists in knee osteoarthritis found evidence for anti-inflammatory effects and cartilage-protective signaling independent of weight change, largely from animal models using older GLP-1 drugs like liraglutide. TRIUMPH-4 was not designed to separate a weight effect from a direct joint effect, so the honest answer is that both are plausible contributors and the trial cannot tell you the split.

The side effect that stands out

The gastrointestinal side effect profile was typical: nausea in 38–43% of the retatrutide arms versus 10.7% on placebo, diarrhea in 33–35% versus 13.4%. Those numbers are in line with what retatrutide shows in its obesity-only trials.

Dysesthesia was not. That is the clinical term for an abnormal skin sensation, tingling, burning, or numbness without an obvious cause, and it showed up in 8.8% of the 9mg group and 20.9% of the 12mg group, against 0.7% on placebo. That rate is high enough to be worth flagging on its own, and it tracked with discontinuations: 12.2% of the 9mg group and 18.2% of the 12mg group stopped the drug due to side effects, compared with 4% on placebo. Whether dysesthesia is specific to this population, people with osteoarthritis often have some baseline nerve involvement in the affected limb, or is a broader retatrutide signal that shows up more clearly at higher doses, is not yet clear from the topline release.

What this does not tell us yet

This is topline data from a company press release, not a peer-reviewed paper. The full publication, expected at a future medical conference alongside a journal submission, should include patient-level function scores (the WOMAC also has stiffness and physical function subscales not detailed in the topline release), imaging data on whether joint structure changed at all, and a clearer breakdown of who discontinued and why.

Retatrutide also is not approved for anything yet. Eli Lilly is expected to file for obesity approval sometime in 2026, with six additional Phase 3 readouts due the same year. Osteoarthritis would need its own regulatory path after that, likely years out. For a full rundown of where the broader retatrutide program stands, see what retatrutide is and how it works, and for the diabetes and cardiovascular arms of the same trial program, see TRIUMPH-2 and TRIUMPH-3.

Where this leaves someone with knee osteoarthritis today

Retatrutide is not available outside a clinical trial. For someone dealing with both excess weight and knee osteoarthritis right now, the treatment options remain what they were before TRIUMPH-4: an approved GLP-1 like tirzepatide or semaglutide, physical therapy, weight-bearing exercise modification, and standard pain management, with joint replacement as the eventual option for severe cases. The TRIUMPH-4 data is a strong signal that significant weight loss from any effective drug in this class likely helps knee pain to some degree. It is not evidence that retatrutide specifically, once approved, will be prescribed as an osteoarthritis treatment on its own.