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Retatrutide TRIUMPH-4: 28.7% Weight Loss, New Safety Signal

Retatrutide's first Phase 3 win: TRIUMPH-4 shows 28.7% weight loss in obesity plus knee OA, with a novel dysesthesia signal at the highest dose.

May 19, 2026 · 3 min read

Eli Lilly's triple-agonist retatrutide produced its first Phase 3 win in TRIUMPH-4, a trial enrolling adults with obesity and knee osteoarthritis. The 12 mg highest dose delivered a mean 28.7% body weight loss at 68 weeks — equivalent to roughly 71 pounds from a baseline of approximately 250 — alongside a 75.8% reduction in knee pain scores. A previously unreported safety signal, dysesthesia (abnormal skin sensations), emerged at the highest dose and will require close monitoring in subsequent readouts.

What happened

TRIUMPH-4 was the first Phase 3 trial in the TRIUMPH program to report topline results, doing so in December 2025. The trial enrolled adults with obesity or overweight (BMI ≥27 with comorbidities) and moderate-to-severe knee osteoarthritis, without type 2 diabetes.

Across three doses:

  • 12 mg: mean 28.7% weight loss at 68 weeks; 75.8% reduction in Western Ontario McMaster Universities Osteoarthritis Index (WOMAC) pain scores
  • 8 mg: 22.8% weight loss; substantial knee pain reduction
  • Placebo: 2.9% weight loss; minimal pain change

In participants with prediabetes at baseline, approximately 72% reverted to normoglycemia — a figure in line with the prediabetes reversal seen in the tirzepatide SURMOUNT-1 trial.

The dysesthesia signal — sensations of tingling, numbness, or abnormal skin sensitivity — emerged specifically at the 12 mg dose. The rate was higher than observed in the GLP-1/GIP dual-agonist class. Its mechanism is not yet characterized; the glucagon receptor component is a plausible suspect given that glucagon signaling extends into the peripheral nervous system.

Why it matters

TRIUMPH-4 establishes that retatrutide's Phase 2 efficacy signal (24.2% weight loss at 48 weeks) translates to Phase 3 at a longer duration and in a broader trial population. The 28.7% figure is the highest ever reported in a randomized Phase 3 trial of any GLP-1-class compound.

The knee osteoarthritis data is also notable: 75.8% pain reduction exceeds what weight loss alone would predict at this timescale, suggesting a direct anti-inflammatory or joint-protective mechanism — a hypothesis that would be biologically plausible via the glucagon pathway's effects on fat tissue inflammation.

For context on how retatrutide's glucagon component works and why the energy-expenditure mechanism is central to its efficacy, see our retatrutide glucagon paradox explainer.

What to watch

TRIUMPH-1 — the pivotal general obesity trial without an OA requirement — is expected to report topline results in Q2-Q3 2026. That readout will be the primary dataset supporting any FDA NDA submission for the obesity indication and will give the clearest picture of real-world efficacy independent of the OA population.

The dysesthesia signal is the key safety question going forward. If it replicates across TRIUMPH-1 and TRIUMPH-2 (type 2 diabetes), the FDA will want to understand mechanism, reversibility, and dose dependency before approving the 12 mg dose — or may require a label warning similar to the thyroid signal on GLP-1s generally. For the realistic timeline to approval, see the retatrutide FDA approval timeline post.

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