Retatrutide (GLP-3): The Triple Receptor Game-Changer
The first triple receptor agonist activating GLP-1, GIP, and glucagon receptors simultaneously. Phase 2 trials show effects significantly beyond semaglutide and tirzepatide.
What Makes Retatrutide Different
Retatrutide (originally LY3437943) is the first peptide to simultaneously activate three different receptors in the metabolic regulation cascade: GLP-1 (like semaglutide), GIP (like the second receptor in tirzepatide), and glucagon (a third pathway entirely new to this class). Each pathway contributes distinct effects: GLP-1 reduces appetite and slows gastric emptying, GIP enhances insulin sensitivity and metabolic flexibility, and glucagon receptor activation increases energy expenditure. The triple-pathway approach produces effects in clinical trials that significantly exceed those of single or dual-receptor agonists.
Phase 2 Clinical Data
Eli Lilly's Phase 2 trials demonstrated weight reductions of 24% over 48 weeks at the highest dose tested (12mg weekly). For comparison, semaglutide produces approximately 15% reduction over similar timeframes, and tirzepatide approximately 22%. Glucose control improvements were equally significant. The trials used a titration schedule starting at 2mg/week and increasing every 4 weeks to manage GI side effects, which were the primary tolerability concern.
Why It's Sometimes Called GLP-3
The 'GLP-3' nickname is informal — there's no actual GLP-3 receptor. The term emerged as a way to communicate the next-generation nature of triple-receptor agonism, building on the GLP-1 (semaglutide) and 'GLP-1 plus' (tirzepatide as dual GLP-1/GIP) naming convention. The compound itself is a synthetic peptide engineered for multi-receptor binding affinity, not a third evolution of the GLP family.
Research Dosing Protocol
Phase 2 trials titrate from 2mg/week to 4mg/week to 6mg/week to 8mg/week to 12mg/week, with each dose held for 4 weeks before titrating up. This slow escalation is essential — rapid titration produces significant GI effects (nausea, occasional vomiting) that often resolve with adaptation. Most research subjects tolerate 6-8mg/week well, with the 12mg/week dose reserved for those with high tolerance and continued response need.
Side Effects and Considerations
GI effects (nausea, occasional vomiting, constipation, diarrhea) are the most common. They typically peak in the first 1-2 weeks of each dose increase and resolve. Appetite reduction is the intended effect and can be substantial — research subjects often need to consciously eat enough protein to maintain lean mass. Glucagon receptor activation produces a small heart rate elevation (~3-7 bpm) that should be monitored. As a research compound, regular monitoring of liver enzymes and metabolic panels is recommended.
What to Verify in a Supplier
Retatrutide is one of the most-counterfeited peptides on the market due to high demand. HPLC purity verification at >98% is essential — anything lower may be undosed or contain impurities. Mass spectrometry confirmation of the molecular weight (3947 Da) verifies you have the correct compound. Net peptide content testing matters more for retatrutide than most peptides because dose precision matters at low milligram quantities. Suppliers using Janoshik or Eurofins for testing are most reliable.