In June 2023, the New England Journal of Medicine published a 48-week Phase 2 trial of retatrutide, a triple agonist of the GIP, GLP-1, and glucagon receptors. The headline number — a mean body-weight reduction of 24.2% at the highest dose — was the largest reported in a randomized obesity trial of this length to date. It is worth reading past the headline.

The trial

The study randomized 338 adults with obesity to once-weekly subcutaneous retatrutide or placebo across escalating dose groups over 48 weeks. The response was clearly dose-dependent. At 48 weeks the least-squares mean change in body weight was −8.7% at 1 mg, −17.1% at 4 mg, −22.8% at 8 mg, and −24.2% at 12 mg, against −2.1% for placebo.

The curve had not flattened

The detail most worth noting is in the shape of the trajectory: participants on retatrutide were still losing weight when treatment stopped at 48 weeks. The authors observed that a plateau had not yet been reached — meaning the 24.2% figure represents a floor on efficacy for that duration, not a ceiling. That open-ended trajectory is part of why the result drew so much attention.

Beyond body weight

Companion Phase 2 work extended the picture. In adults with type 2 diabetes, retatrutide reduced HbA1c by roughly 2.2 percentage points at the highest dose, with about 82% of higher-dose participants reaching HbA1c at or below 6.5%. A separate liver-fat substudy reported reductions in hepatic steatosis exceeding 80% at 24 weeks. The glucagon arm — absent from GLP-1 and dual GIP/GLP-1 agonists — is the leading hypothesis for the added effect on energy expenditure and hepatic lipid handling.

What it means for research

These are Phase 2 results in a defined investigational context, and the Phase 3 TRIUMPH program is what will ultimately characterize efficacy and safety. For laboratory and pre-clinical researchers, the value of well-characterized reference material is exactly this kind of traceability: the ability to connect an observed effect back to a documented compound, lot, and purity.