On December 11, 2025, Eli Lilly announced the first Phase 3 results for retatrutide—a first-in-class triple hormone receptor agonist targeting GLP-1, GIP, and glucagon simultaneously. The results exceeded analyst expectations and establish retatrutide as potentially the most powerful weight-loss medication ever tested.
How Retatrutide Is Different
Retatrutide targets three hormone receptors simultaneously:
| Receptor | Function | Why It Matters |
|---|---|---|
| GLP-1 | Reduces appetite, slows gastric emptying | The foundation of Ozempic/Wegovy |
| GIP | Enhances insulin secretion, improves lipid metabolism | Added by Mounjaro/Zepbound (dual agonist) |
| Glucagon | Increases energy expenditure, boosts fatty acid oxidation | The NEW third component—unique to retatrutide |
The glucagon component is key: while GLP-1 and GIP primarily reduce food intake, glucagon activation increases energy expenditure and may help reduce liver fat more effectively.
How It Compares to Current Drugs
Retatrutide delivered approximately 37% more weight loss than tirzepatide and nearly double the weight loss of semaglutide.
The TRIUMPH-4 Trial
TRIUMPH-4 (NCT05931367) was a Phase 3, randomized, double-blind, placebo-controlled trial evaluating retatrutide in adults with obesity or overweight plus knee osteoarthritis.
| Participants | 445 adults (no diabetes) |
| Baseline weight | 248.5 lbs average |
| Doses tested | 9 mg and 12 mg weekly |
| Duration | 68 weeks |
| Co-primary endpoints | Weight loss + osteoarthritis pain improvement |
Primary Weight Loss Results
| Dose | Weight Loss (%) | Weight Loss (lbs) |
|---|---|---|
| Retatrutide 9 mg | 26.4% | 64.2 lbs |
| Retatrutide 12 mg | 28.7% | 71.2 lbs |
| Placebo | 4.2% | ~10 lbs |
On the highest dose, 23.7% of patients achieved ≥35% weight loss—approaching bariatric surgery results.
Osteoarthritis Pain Results
Retatrutide also demonstrated dramatic improvement in knee pain, measured by the WOMAC pain score:
- 9 mg dose: 75.8% reduction in pain (-4.5 points)
- 12 mg dose: 62.6% reduction in pain (-3.7 points)
- Placebo: 40.3% reduction (-2.4 points)
Notably, 14.1% of patients on 9 mg and 12.0% on 12 mg were completely free of knee pain at 68 weeks, compared to 4.2% on placebo.
Cardiovascular Risk Markers
Retatrutide also improved key CV risk markers:
- Reduced non-HDL cholesterol
- Reduced triglycerides
- Reduced high-sensitivity C-reactive protein (hsCRP)
- Systolic blood pressure: -14.0 mmHg at highest dose
Safety and Side Effects
The side effect profile was similar to other incretin medications, though GI effects were more common:
| Side Effect | 9 mg | 12 mg | Placebo |
|---|---|---|---|
| Nausea | 38.1% | 43.2% | 10.7% |
| Diarrhea | 34.7% | 33.1% | 13.4% |
| Constipation | 21.8% | 25.0% | 8.7% |
| Vomiting | 20.4% | 20.9% | 0.0% |
| Decreased appetite | 19.0% | 18.2% | 9.4% |
| Dysesthesia (tingling) | 8.8% | 20.9% | 0.7% |
Dysesthesia (abnormal sensation, often tingling or numbness) was notably more common with retatrutide than other GLP-1 medications. While generally mild and rarely leading to discontinuation, this is something to watch in future trials.
Discontinuation Rates
Discontinuation rates due to adverse events were:
- 9 mg: 12.2%
- 12 mg: 18.2%
- Placebo: 4.0%
Lilly noted these discontinuations were "highly correlated with baseline BMI and included discontinuations for perceived excessive weight loss"—meaning some patients left because they felt they were losing too much weight.
Phase 2 Recap: What We Already Knew
The Phase 2 trial (NEJM, August 2023) had already shown impressive results in patients with obesity:
- 24.2% weight loss at 48 weeks with the 12 mg dose
- In patients with type 2 diabetes: 16.9% weight loss, 2.2% HbA1c reduction
- 77-82% of diabetic patients achieved euglycemia (HbA1c ≤6.5%)
The Phase 3 results actually exceeded Phase 2, which is unusual—typically Phase 3 results are slightly lower than Phase 2 due to larger, more diverse populations.
What's Next for Retatrutide
Lilly has 7 additional Phase 3 trials expected to report results in 2026, studying retatrutide for:
- Obesity (TRIUMPH-1)
- Type 2 diabetes
- Obstructive sleep apnea
- Chronic low back pain
- Cardiovascular outcomes
- Renal outcomes
- Metabolic dysfunction-associated steatotic liver disease (MASLD/MASH)
Expected FDA approval: 2027 (GlobalData projection)
Market Context
If approved, retatrutide would enter a rapidly expanding market:
- Zepbound (tirzepatide) generated $3.6 billion in Q3 2025 alone
- GlobalData projects retatrutide sales of $15.6 billion by 2031
- Lilly is also awaiting FDA decision on oral orforglipron (GLP-1 pill)
What We Don't Know Yet
- Long-term safety—68 weeks is promising but multi-year data is needed
- Cardiovascular outcomes—will triple agonism provide CV mortality benefit like semaglutide?
- Muscle preservation—with even greater weight loss, is lean mass loss a bigger concern?
- Maintenance dosing—Lilly is testing a 4 mg maintenance dose in some trials
- "Too much weight loss" concern—some discontinuations were for excessive weight loss; how will this be managed?
Retatrutide's first Phase 3 results establish it as potentially the most effective weight-loss medication ever tested. The 28.7% average weight loss (71 lbs) exceeds both semaglutide (~15%) and tirzepatide (~21%), with additional benefits for osteoarthritis pain and cardiovascular risk markers. The triple-agonist mechanism—adding glucagon to GLP-1 and GIP—appears to meaningfully increase efficacy. However, GI side effects and discontinuation rates were higher than current drugs, and the new dysesthesia signal requires monitoring. With 7 more Phase 3 readouts expected in 2026 and projected FDA approval in 2027, retatrutide could represent the next major leap in obesity treatment for patients who need more than current medications can provide.
Sources
- Eli Lilly. "Lilly's triple agonist, retatrutide, delivered weight loss of up to an average of 71.2 lbs along with substantial relief from osteoarthritis pain in first successful Phase 3 trial." Press release. December 11, 2025.
- Jastreboff AM, et al. "Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial." New England Journal of Medicine. 2023;389(6):514-26.
- Rosenstock J, et al. "Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes." The Lancet. 2023;402(10401):529-44.
- Clinical Trials Arena. "Lilly's triple G agonist boasts 28.7% weight loss in Phase III trial." December 2025.
- Fierce Biotech. "Eli Lilly's obesity triple threat smashes efficacy expectations in phase 3." December 2025.
- ClinicalTrials.gov. TRIUMPH-4 (NCT05931367).
- PMC. "Triple Agonism Based Therapies for Obesity." 2025.
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