Retatrutide

Retatrutide is a synthetic triple receptor agonist developed by Eli Lilly that simultaneously activates three receptors: GLP-1 (glucagon-like peptide-1), GIP (glucose-dependent insulinotropic polypeptide), and the glucagon receptor. It represents the third generation of injectable weight-loss peptides following GLP-1 mono-agonists (semaglutide) and GLP-1/GIP dual agonists (tirzepatide). As of 2025, retatrutide is in Phase 3 clinical trials and has not been approved by the FDA for any indication.

The addition of glucagon receptor agonism is what distinguishes retatrutide from tirzepatide and is the key driver of its superior efficacy. Glucagon increases hepatic glucose output and raises basal metabolic rate by stimulating energy expenditure in the liver and adipose tissue. This thermogenic component means retatrutide attacks obesity through three distinct mechanisms simultaneously: reduced appetite (GLP-1/GIP), improved glucose metabolism and insulin sensitivity (GLP-1/GIP), and increased energy expenditure (glucagon). The three-way synergy produces weight loss that significantly exceeds either predecessor.

Phase 2 results published in the New England Journal of Medicine in 2023 demonstrated mean body weight reductions of 17.5% at 4 mg, 22.8% at 8 mg, and 24.2% at 12 mg over 48 weeks, the largest weight loss ever documented in a pharmaceutical trial at the time of publication. Approximately 26% of participants at the highest dose achieved 40%+ body weight reduction, a result never previously seen outside of bariatric surgery. These numbers significantly exceed tirzepatide's 22.5% and semaglutide's 14.9% at maximum doses in comparable timeframes.

Reconstitution follows the same procedure as other injectable peptides. A 10 mg vial with 2 mL of bacteriostatic water gives a concentration of 5 mg/mL. At a 4 mg dose, you draw 0.8 mL (80 units on a U-100 syringe). Inject subcutaneously in the abdomen, outer thigh, or upper arm. Titration is gradual, 4 weeks minimum at each dose step, to allow GI tolerance to develop. Reconstituted retatrutide should be stored at 2–8°C and used within 28 days.

The glucagon component introduces a nuance absent from semaglutide and tirzepatide: mild elevations in blood glucose, particularly at the beginning of treatment. The GLP-1 component largely counteracts this by driving insulin secretion, but the net effect may require monitoring in people with diabetes or prediabetes. GI side effects (nausea, vomiting, diarrhea, constipation) follow the same pattern as other GLP-1 agonists, dose-dependent, front-loaded during titration, improving over time with slow titration and dietary modifications.