TRIUMPH-4 and the Knee Osteoarthritis Surprise: What the First Phase 3 Retatrutide Trial Actually Showed
If you’ve struggled with knee pain alongside excess weight, the December 2025 TRIUMPH-4 results are the kind of data that turn heads. Participants on the highest dose of retatrutide lost an average of 28.7% of their body weight at 68 weeks — and reported a 75.8% reduction in knee osteoarthritis pain on a standard pain scale. Roughly one in eight participants ended the trial completely free of knee pain. Here’s what the trial actually showed and what it does and does not mean.
TRIUMPH-4 was the first Phase 3 retatrutide trial to report a successful primary endpoint. It enrolled 445 adults with obesity or overweight and knee osteoarthritis, randomized 1:1:1 to retatrutide 9 mg, retatrutide 12 mg, or placebo for 68 weeks of treatment.
According to Eli Lilly’s December 2025 announcement and subsequent peer-reviewed coverage, the 12 mg dose produced a mean 28.7% body-weight reduction (about 71.2 lbs) and a 4.5-point reduction on the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain scale — equivalent to a 75.8% pain decrease. A post-hoc analysis showed that 12.0% of the 12 mg arm and 14.1% of the 9 mg arm were completely free of knee pain at week 68, compared with 4.2% on placebo.
What TRIUMPH-4 Was Designed to Test
TRIUMPH-4 is a randomized, double-blind, placebo-controlled Phase 3 trial of retatrutide in adults with obesity or overweight and knee osteoarthritis. It is registered on ClinicalTrials.gov as NCT05931367 and is part of Eli Lilly’s broader TRIUMPH Phase 3 program.
The trial enrolled 445 adults and randomized them 1:1:1 to retatrutide 9 mg, retatrutide 12 mg, or matching placebo, with treatment lasting 68 weeks. Two co-primary outcome categories were tested: change in body weight from baseline, and change in WOMAC pain score from baseline. WOMAC is the standard validated questionnaire used in osteoarthritis trials and assesses pain across activities such as walking, stair climbing, sleeping, and rest.
Importantly, TRIUMPH-4 was not designed to detect joint structural change on imaging. The pain endpoint reflects the patient-reported experience of osteoarthritis pain, not a measure of cartilage thickness, joint space, or radiographic progression.
The Headline Numbers
Three numbers carried the December 2025 TRIUMPH-4 announcement.
Mean body weight loss of 28.7% at week 68 in the 12 mg retatrutide arm, equivalent to approximately 71.2 lbs. The 9 mg arm produced a smaller but still substantial reduction. Both retatrutide doses met the primary weight-loss endpoint and key secondary endpoints versus placebo.
A 4.5-point reduction in WOMAC pain score in the 12 mg arm — a 75.8% decrease relative to baseline. Both retatrutide doses also produced statistically significant improvements in physical function as measured on the WOMAC subscale.
12.0% of participants in the 12 mg arm and 14.1% in the 9 mg arm were completely pain-free at week 68 in a post-hoc analysis, compared with 4.2% on placebo. The 9 mg arm showing a slightly higher pain-free rate than the 12 mg arm at the same primary endpoint is a reminder that pain response is not strictly dose-linear, and that any single sub-analysis should be read alongside the full dataset rather than on its own.
Why the Knee-Pain Result Surprised People
Weight loss helps knee osteoarthritis pain. That has been clinically established for decades — every kilogram of body weight lost reduces the load across the tibiofemoral joint by several kilograms during walking. What surprised the field about TRIUMPH-4 was the magnitude of the pain effect, not its direction.
Most weight-loss interventions in adults with knee osteoarthritis produce pain reductions in the 20% to 40% range relative to baseline over six to eighteen months of follow-up. A 75.8% pain reduction in the 12 mg arm is well outside that range. The most plausible explanation is the depth of the weight loss itself: 28.7% mean body-weight reduction is roughly two to three times what behavioral and lifestyle interventions typically achieve in trials of this duration.
There is an open question about whether retatrutide may also produce pain effects that are not purely mediated by weight loss — for example, through reduced systemic inflammation. TRIUMPH-4 was not designed to answer that question definitively, and Lilly has not made claims along those lines. The data is consistent with a primarily weight-mediated mechanism, but does not rule out additional contributions.
How the Pain Reduction Compares to Standard Osteoarthritis Treatments
Two reference points matter for context.
Oral nonsteroidal anti-inflammatory drugs (NSAIDs) — the most commonly prescribed pharmacologic treatment for knee osteoarthritis pain — typically produce pain reductions of 15% to 25% over short-term treatment periods. Intra-articular corticosteroid injections produce larger short-term reductions but are limited by duration and frequency-of-use considerations.
Total knee replacement, the most aggressive option, eliminates osteoarthritis pain in roughly 80% of patients but carries surgical risk, recovery time, and lifetime prosthesis durability considerations.
TRIUMPH-4 does not replace any of these treatments. What it does is establish — for the first time in a Phase 3 obesity-medication trial — that an injectable obesity drug can deliver a pain reduction in the same range as more invasive interventions, in a population for whom both excess weight and joint pain are central clinical concerns.
The Body-Weight and Structural-Pain Connection
The TRIUMPH-4 result deepens an emerging picture about the connection between obesity medicine and conditions that have historically been treated as orthopedic, rheumatologic, or surgical problems.
Knee osteoarthritis is the most common joint disease worldwide. It is also one of the leading drivers of disability and reduced quality of life in adults with obesity. Until recently, the standard recommendation for adults with both conditions was lifestyle modification first, NSAIDs and physical therapy second, intra-articular injections third, and joint replacement at the end of the line. The implicit assumption was that medication-driven weight loss would not be deep or durable enough to materially affect joint pain.
TRIUMPH-4 challenges that assumption. It does not replace any of the existing treatment options, but it suggests that obesity-medication-driven weight loss may have a more central role in osteoarthritis treatment planning than previously assumed.
For more on how this trial fits into the broader retatrutide research picture, see our retatrutide and knee osteoarthritis page and the Phase 3 TRIUMPH program research summary.
Safety and Tolerability in TRIUMPH-4
The adverse-event profile in TRIUMPH-4 was consistent with the incretin-based therapy class.
The most common adverse events in the retatrutide arms were gastrointestinal: nausea (38.1% on 9 mg, 43.2% on 12 mg, versus 10.7% on placebo), diarrhea (34.7% and 33.1% versus 13.4%), constipation (21.8% and 25.0% versus 8.7%), and vomiting (20.4% and 20.9% versus a smaller proportion on placebo).
These rates are higher than what has been reported in some earlier retatrutide trials, partly because TRIUMPH-4 used a longer treatment period and partly because gastrointestinal adverse events are typically reported cumulatively rather than as point prevalence. Most events were transient and concentrated during the dose-escalation period, which is the consistent pattern across incretin-based therapies.
For broader background on retatrutide tolerability and how side effects have been reported across the trial program, see our retatrutide side effects page and the safety and tolerability research summary.
What TRIUMPH-4 Doesn’t Tell Us
TRIUMPH-4 was a positive Phase 3 readout, but the questions it does not answer are as important as the ones it does.
It does not tell us whether retatrutide changes the structural progression of knee osteoarthritis. The trial measured pain and function, not joint imaging.
It does not tell us whether retatrutide reduces the long-term need for total knee replacement. That question requires longer follow-up than 68 weeks of treatment can provide.
It does not tell us how retatrutide compares head-to-head against other obesity medications in adults with knee osteoarthritis. TRIUMPH-4 was placebo-controlled, not active-comparator. Comparisons against tirzepatide or semaglutide will come from other trials.
And it does not, by itself, deliver an FDA-approved indication for knee osteoarthritis. A potential knee OA label would require regulatory submission, review, and approval — and would more likely be carried as a comorbidity within a broader weight-management label than as a standalone indication.
How TRIUMPH-4 Reframes Obesity Medicine
It is worth being clear about what is and isn’t new here.
What is not new: the relationship between excess body weight and knee osteoarthritis pain. That relationship has been understood for decades.
What is new: the demonstration that a single injectable medication can deliver a 28.7% mean body-weight reduction over 68 weeks in adults with obesity and knee osteoarthritis, and that this depth of weight loss translates into a pain reduction in the range previously associated with more invasive interventions.
TRIUMPH-4 does not, on its own, change clinical practice. Retatrutide is not approved or available. But it is a Phase 3 result with a clinically meaningful effect on a condition that millions of adults live with — and that fact is part of why retatrutide is being watched as closely as it is. For the broader picture of how this fits into the next generation of obesity medicine, see our overview of how retatrutide works.
Stay Updated
If you’d like a single, plain-English summary the moment TRIUMPH-1 reports — the trial that will most likely anchor a future U.S. weight-management approval for retatrutide — you can join our retatrutide updates list. One email per major milestone, no marketing.
Disclaimer
Retatrutide is an investigational medication. It is not approved by the FDA for any indication, including knee osteoarthritis, and is not commercially available. This post is educational and should not be interpreted as medical advice or as a recommendation for any specific treatment. For information about how our content is sourced and reviewed, see our editorial policy and medical review policy.
FAQ SECTION
How much weight did participants lose in TRIUMPH-4?
Participants in the retatrutide 12 mg arm lost a mean 28.7% of body weight at 68 weeks, equivalent to approximately 71.2 lbs. The 9 mg arm produced a smaller but still substantial reduction. Both retatrutide doses met the primary weight-loss endpoint versus placebo. Lilly reported these as the highest weight-loss figures from any retatrutide trial to date.
How is WOMAC pain measured?
WOMAC is the Western Ontario and McMaster Universities Osteoarthritis Index, a standardized questionnaire widely used in knee osteoarthritis trials. It measures self-reported pain during activities such as walking, stair climbing, sitting, lying down, and rest. In TRIUMPH-4, the 12 mg retatrutide arm produced a 4.5-point reduction in the WOMAC pain subscale — a 75.8% decrease relative to baseline.
How many TRIUMPH-4 participants ended pain-free?
In a post-hoc analysis at week 68, 12.0% of participants in the retatrutide 12 mg arm and 14.1% in the 9 mg arm were classified as completely free of knee pain, compared with 4.2% on placebo. The pain-free rate being slightly higher in the 9 mg arm than the 12 mg arm is a reminder that pain response is not strictly dose-linear and that single sub-analyses should be read in the context of the full dataset.
Was retatrutide compared to NSAIDs or other osteoarthritis treatments in TRIUMPH-4?
No. TRIUMPH-4 was placebo-controlled, meaning retatrutide was compared against a matching placebo injection rather than against NSAIDs, intra-articular corticosteroid injections, or surgical interventions. Cross-trial comparisons against those treatments are unreliable because populations, follow-up durations, and outcome measures differ. TRIUMPH-4 establishes a placebo-controlled effect size, not a comparative effectiveness claim.
Does TRIUMPH-4 mean retatrutide is approved for knee osteoarthritis?
No. TRIUMPH-4 is a positive Phase 3 readout but does not constitute regulatory approval. Retatrutide remains investigational and is not commercially available for any indication. A potential U.S. label would more likely cover weight management with knee osteoarthritis as a comorbidity than a standalone osteoarthritis indication, and would require regulatory submission, review, and approval.
How big was the TRIUMPH-4 trial?
TRIUMPH-4 enrolled 445 adults with obesity or overweight and knee osteoarthritis, randomized 1:1:1 to retatrutide 9 mg, retatrutide 12 mg, or placebo for 68 weeks of treatment. By comparison, TRIUMPH-1 — the larger pivotal Phase 3 retatrutide trial in adults with obesity who do not have type 2 diabetes — enrolled approximately 2,300 participants over 80 weeks.
