Retatrutide is the most recent addition to the incretin-agonist class and the first molecule that activates three receptors simultaneously: GLP-1, GIP, and glucagon. Eli Lilly’s phase 2 results, published in NEJM in 2023, produced the largest weight-loss endpoint yet observed in this class. This article walks through what the data actually shows and what the triple mechanism implies.
The molecule
Retatrutide is a synthetic peptide engineered to activate three distinct G-protein-coupled receptors: GLP-1, GIP, and the glucagon receptor. The sequence builds on the dual-agonist framework of tirzepatide but adds a glucagon arm that fundamentally changes the metabolic profile. Plasma half-life is approximately 6 days, supporting weekly dosing.
Why adding glucagon receptor activation matters
Glucagon is typically associated with raising blood glucose — the opposite of what an anti-diabetic peptide should do. But glucagon also drives energy expenditure: it stimulates hepatic lipid oxidation, increases resting metabolic rate, and promotes thermogenesis. In a balanced multi-agonist, the GLP-1 arm controls appetite, the GIP arm sensitises insulin, and the glucagon arm lifts energy output.
The result: retatrutide acts on both sides of the energy balance equation. GLP-1 and GIP cut intake; glucagon raises output. The earlier dual agonists (tirzepatide, semaglutide) primarily addressed intake.
Phase 2 trial — the headline numbers
Jastreboff et al., NEJM 2023 (phase 2, 338 adults with obesity, 48 weeks):
| Dose | Weight loss at 48 weeks |
|---|---|
| Placebo | 2.1% |
| Retatrutide 1 mg weekly | 8.7% |
| Retatrutide 4 mg weekly | 17.1% |
| Retatrutide 8 mg weekly | 22.8% |
| Retatrutide 12 mg weekly | 24.2% |
The 12 mg arm produced 24.2% mean body weight reduction — the largest endpoint published for any anti-obesity peptide. Notably, the weight loss curve had not plateaued at 48 weeks, suggesting continued benefit with longer exposure. Phase 3 trials are underway.
Cardiometabolic markers improved beyond weight
The trial reported improvements across multiple cardiometabolic endpoints:
- Reduced fasting glucose and HbA1c
- Lower triglycerides and LDL cholesterol
- Improved blood pressure
- Reduced liver fat (proton MRS measurement)
The liver fat reduction is mechanistically tied to glucagon receptor activation, which drives hepatic lipid oxidation.
Side effects in the phase 2 record
The adverse event profile matches the broader class — nausea, diarrhoea, constipation — but with a dose-dependent pattern. At the 12 mg dose, gastrointestinal events were more frequent. Importantly, the glucagon component did not produce the hyperglycaemia that pure glucagon agonism would cause: the GLP-1/GIP arms compensate.
Where retatrutide sits in the class hierarchy
| Compound | Receptors | Best published endpoint |
|---|---|---|
| Semaglutide | GLP-1 | ~14.9% at 68 weeks |
| Tirzepatide | GLP-1 + GIP | ~22.5% at 72 weeks |
| Retatrutide | GLP-1 + GIP + glucagon | ~24.2% at 48 weeks (phase 2) |
See our detailed three-way comparison.
What remains unknown
- Long-term safety beyond 48 weeks
- Durability after discontinuation
- Phase 3 head-to-head results vs tirzepatide
- Cardiovascular outcomes (CVOT trials pending)
Research handling
Retatrutide is supplied lyophilised. Reconstitution in bacteriostatic water at 5–10 mg/mL is typical. The triple-agonist architecture is structurally complex — batch-specific CoA verification is non-negotiable for reliable stability data.
Chempeptides supplies HPLC-verified retatrutide with batch CoA — see the catalogue for current availability.
Research use only. Phase 2 data summarised from published clinical research.