If Zepbound rewrote the playbook for injectable weight loss drugs, retatrutide might rewrite it again. Eli Lilly's triple receptor agonist delivered 28.7% body weight loss in Phase 2 trials — the highest figure ever recorded in an obesity drug study. Here's everything we know about the drug, the science, and when it might be available.
What Is Retatrutide?
Retatrutide (development code LY3437943) is a once-weekly injectable that simultaneously activates three hormone receptors:
- GLP-1 (glucagon-like peptide-1) — the same target as Wegovy, Ozempic, and Foundayo
- GIP (glucose-dependent insulinotropic polypeptide) — the second target in Zepbound/Mounjaro
- Glucagon receptor — the new addition that makes retatrutide a "triple agonist"
Think of it this way: Wegovy hits one target. Zepbound hits two. Retatrutide hits all three. Each additional receptor activation adds a distinct metabolic benefit that the others don't provide alone.
Why Adding Glucagon Matters
The glucagon receptor is the piece that makes retatrutide genuinely novel. While GLP-1 and GIP primarily work through appetite suppression and improved insulin signaling, glucagon receptor activation drives two additional effects:
- Increased energy expenditure. Glucagon stimulates the liver to burn fat for energy and may increase resting metabolic rate — partially countering the metabolic slowdown that typically accompanies weight loss
- Dramatic liver fat reduction. In Phase 2 data, retatrutide reduced liver fat by up to 37%, far exceeding what GLP-1 or GIP/GLP-1 agonists achieve. This has major implications for patients with non-alcoholic fatty liver disease (NAFLD/MASH)
The concern with glucagon activation has always been blood sugar: glucagon raises blood glucose, which seems counterproductive in a drug that might be used by people with diabetes. Retatrutide's design manages this by balancing glucagon's glucose-raising effect with the glucose-lowering effects of GLP-1 and GIP. In trials, blood sugar control actually improved.
Phase 2 Results: Record-Breaking Weight Loss
The Phase 2 trial enrolled 338 adults with obesity and tested multiple doses over 48 weeks. The results at the highest dose (12 mg):
- 28.7% mean body weight loss among completers at 48 weeks
- 24.2% in the intention-to-treat analysis
- 100% of participants at the highest dose lost at least 5% body weight
- 83% lost at least 15% body weight
- 63% lost at least 20% body weight
To put 28.7% in perspective: for a 250 lb patient, that's approximately 72 lbs of weight loss in under a year.
How Retatrutide Compares to Everything Else
| Drug | Mechanism | Peak Weight Loss | Phase | Earliest Availability |
|---|---|---|---|---|
| Retatrutide | GLP-1/GIP/glucagon | 28.7% | Phase 3 | 2027–2028 |
| CagriSema | GLP-1 + amylin | 22.7% | NDA filed | Q1 2027 |
| Zepbound | GLP-1/GIP | 22.5% | Approved | Available now |
| Wegovy | GLP-1 | 15.0% | Approved | Available now |
| Foundayo | GLP-1 (oral small molecule) | 12.4% | Approved | Available now |
For more details on CagriSema, see our CagriSema guide. For the complete pipeline picture, check our GLP-1 drug pipeline overview.
Phase 3: The TRIUMPH Program
Lilly's Phase 3 program for retatrutide is called TRIUMPH, comprising at least four major trials:
- TRIUMPH-1: Retatrutide in adults with type 2 diabetes
- TRIUMPH-2: Retatrutide in adults with obesity and type 2 diabetes
- TRIUMPH-3: Retatrutide for MASH (metabolic dysfunction-associated steatohepatitis)
- TRIUMPH-4: Retatrutide in adults with obesity without diabetes — the pivotal obesity trial
Phase 3 results are expected to read out through 2026 and into 2027. If results confirm the Phase 2 data — even at slightly lower efficacy — an NDA filing could come in late 2026 or 2027, with earliest approval in 2027–2028.
Lilly's Third Obesity Drug
If approved, retatrutide would be Eli Lilly's third obesity medication, following Zepbound (tirzepatide) and Foundayo (orforglipron). This gives Lilly a portfolio spanning multiple mechanisms and delivery formats:
- Foundayo: Entry-level oral pill, 12.4% weight loss, $149/mo — easiest access
- Zepbound: Dual agonist injection, 22.5% weight loss, $299–549/mo — strong efficacy
- Retatrutide: Triple agonist injection, ~28.7% weight loss — maximum efficacy
Should You Wait for Retatrutide?
No. If you need treatment now, effective medications are available today. Zepbound and Foundayo are both on the market. Retatrutide is at least 1–2 years from approval, and Phase 3 trials can fail or produce lower results than Phase 2. Start treatment with what's available, and add retatrutide to your options if and when it arrives. See our Foundayo vs Zepbound comparison for help choosing between today's options.
Bottom Line
Retatrutide's 28.7% weight loss in Phase 2 is extraordinary — if Phase 3 confirms even 80% of that figure, it will be the most effective obesity drug ever approved. The triple-agonist mechanism adds liver fat reduction and increased energy expenditure that dual agonists can't match. But it's not here yet. Watch the TRIUMPH trials, expect an NDA by 2027, and treat what you have today while waiting for what's coming tomorrow.
Sources
- Jastreboff AM et al. "Triple-Hormone-Receptor Agonist Retatrutide for Obesity." NEJM. 2023;389:514-526.
- Eli Lilly. "TRIUMPH Phase 3 Clinical Program for Retatrutide." ClinicalTrials.gov. 2025.
- Rosenstock J et al. "Retatrutide Phase 2 Trial in Type 2 Diabetes." The Lancet. 2023.