What is retatrutide?
Retatrutide (development code LY3437943) is a single peptide engineered by Eli Lilly to activate three hormone receptors simultaneously: GLP-1, GIP, and the glucagon receptor. This "triple agonist" approach is a first in medicine - no other approved or late-stage drug targets all three pathways.
While semaglutide (GLP-1 only) and tirzepatide (GLP-1 + GIP) have already transformed obesity medicine, retatrutide adds the third piece of the puzzle: glucagon receptor activation. Glucagon increases energy expenditure - your body burns more calories at rest, even during sleep. Combined with the appetite-reducing effects of GLP-1 and GIP, this triple mechanism has produced the highest weight-loss results ever seen in a clinical trial.
Key facts
- Class: GLP-1/GIP/Glucagon triple agonist
- Developer: Eli Lilly
- Peak weight loss: 28.6% at 48 weeks (Phase 2)
- Current status: Phase 3 (TRIUMPH program)
- Administration: Weekly subcutaneous injection
- Phase 2 doses tested: 1, 4, 8, 12 mg
- Expected approval: Pending Phase 3 results
- Unique feature: First triple-receptor agonist
Three receptors, three pathways, one molecule
What makes retatrutide unique is the combination of three distinct biological mechanisms in a single weekly injection. Each receptor activation contributes differently to weight loss.
GLP-1 Receptor
Appetite suppression. Signals the hypothalamus to reduce hunger. Slows gastric emptying so you feel full longer. This is the same pathway semaglutide uses - the proven foundation of incretin-based weight loss.
GIP Receptor
Metabolic optimization. Enhances insulin sensitivity in fat tissue, promotes healthy fat distribution, and amplifies the satiety signal from GLP-1. This is the pathway tirzepatide added - the dual-agonist advantage.
Glucagon Receptor
Energy expenditure. This is the new piece. Glucagon activation increases resting metabolic rate and stimulates hepatic fat oxidation. Your body burns more stored fat as fuel - even at rest. This is what separates retatrutide from everything before it.
Why the glucagon receptor matters
Previous GLP-1-based treatments reduce caloric intake (you eat less). Glucagon receptor activation increases caloric expenditure (you burn more). This dual approach - eating less AND burning more - is why retatrutide's Phase 2 weight loss exceeded every previous compound. It also may explain a key finding: in the Phase 2 trial, retatrutide patients appeared to lose a higher proportion of visceral (abdominal organ) fat compared to subcutaneous fat, suggesting the glucagon pathway preferentially targets the most metabolically dangerous fat deposits.
Clinical trial data
The Phase 2 trial enrolled 338 adults with obesity (BMI ≥30 or ≥27 with comorbidity). Participants received one of four retatrutide doses or placebo, weekly, for 48 weeks. Published in The New England Journal of Medicine, August 2023.
| Group | Dose | Participants | Weight Loss at 24 Weeks | Weight Loss at 48 Weeks | ≥15% Loss | ≥20% Loss |
|---|---|---|---|---|---|---|
| Placebo | - | 70 | -2.1% | -2.1% | 3% | 1% |
| Retatrutide | 1 mg | 67 | -7.2% | -8.7% | 12% | 3% |
| Retatrutide | 4 mg (escalated) | 68 | -12.9% | -17.1% | 54% | 25% |
| Retatrutide | 8 mg (escalated) | 67 | -16.4% | -22.8% | 75% | 55% |
| Retatrutide | 12 mg (escalated) | 66 | -16.8% | -28.6% | 83% | 63% |
Source: Jastreboff AM, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity. NEJM. 2023;389:514-526. Doses of 4 mg and above used an escalation protocol starting at 2 mg.
Why these numbers are unprecedented
- 28.6% average weight loss at 48 weeks - semaglutide achieved 16.9% at 68 weeks, and tirzepatide achieved 22.5% at 72 weeks
- Weight loss had not yet plateaued at 48 weeks - the curve was still declining, suggesting longer treatment could produce even greater results
- 83% of participants on the highest dose lost at least 15% of body weight - approaching the threshold historically only achievable with bariatric surgery
- These results were achieved in only 48 weeks - both semaglutide and tirzepatide's pivotal trials ran 68-72 weeks to achieve lower peaks
How does retatrutide compare?
Comparing peak results across the three leading GLP-1-era compounds. Note: these are cross-trial comparisons, not head-to-head studies. Trial populations and durations differ.
Semaglutide
- 1 receptor targeted (GLP-1)
- FDA approved (2021)
- CV outcomes data (SELECT)
- Most long-term safety data
- 1/3 patients lost ≥20%
Tirzepatide
- 2 receptors targeted (GLP-1 + GIP)
- FDA approved (2023)
- Lower nausea rate than semaglutide
- Better lean mass preservation
- ~40% of patients lost ≥25%
Retatrutide
- 3 receptors targeted (GLP-1 + GIP + GCG)
- Phase 3 in progress (TRIUMPH)
- Increases energy expenditure
- May preferentially target visceral fat
- 63% of patients lost ≥20%
Side effects & tolerability
The Phase 2 safety profile was similar to other incretin-based therapies. GI side effects were dose-dependent and most common during the titration period.
Side effect rates - Phase 2 (12 mg group vs placebo)
Discontinuation rate due to adverse events: 6% (12 mg group) vs 0% (placebo). Most GI side effects were mild-to-moderate and occurred during dose escalation. The dose-escalation protocol significantly reduced GI severity compared to starting at the full dose.
Monitoring: what Phase 3 is watching for
- Heart rate: Mild increases were observed in Phase 2 (2-4 bpm average), consistent with other GLP-1 agonists. Phase 3 monitors long-term cardiovascular effects.
- Liver enzymes: Small, transient elevations in ALT were noted in some participants. Glucagon receptor activation stimulates hepatic fat oxidation, which can temporarily raise liver enzymes as fat is mobilized.
- Blood glucose: Glucagon raises blood sugar while GLP-1/GIP lower it - the net effect was glucose-neutral in non-diabetic participants. In T2D participants, glucose control improved.
- Bone density: With this degree of weight loss, monitoring for bone mineral density changes is important. Data from Phase 3 will provide more information.
Retatrutide FAQ
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