Retatrutide Phase 2: 24.2% Weight Loss in 48 Weeks

In June 2023, the New England Journal of Medicine published the first randomised, placebo-controlled Phase 2 trial of retatrutide in adults with obesity. At the highest dose of 12 mg once-weekly, participants lost an average of 24.2% of body weight over 48 weeks — the largest weight reduction ever recorded in a Phase 2 peptide weight-loss trial.

The Trial

338 adults with a BMI ≥30 (or ≥27 with a weight-related condition) were randomised to retatrutide at 1 mg, 4 mg, 8 mg, or 12 mg once weekly, or placebo, for 48 weeks. The study was double-blind and placebo-controlled.

Key Results

At the 12 mg dose:

• 24.2% mean body weight reduction from baseline at 48 weeks
• Mean weight loss of approximately 26.4 kg (58 lbs)
• 83% of participants achieved at least 5% weight loss
• 26% achieved ≥25% weight loss — a threshold rarely reached in prior trials

The dose-response was clear: the 4 mg group lost 8.7% and the 8 mg group lost 17.3%, demonstrating that higher doses compound the effect significantly.

"The magnitude of weight loss with retatrutide was unprecedented in a Phase 2 study." — Jastreboff et al., NEJM 2023

Why Three Receptors?

Semaglutide activates one receptor (GLP-1). Tirzepatide activates two (GLP-1 + GIP). Retatrutide activates three: GLP-1, GIP, and glucagon receptors simultaneously. The glucagon receptor component increases basal energy expenditure — the body burns more at rest — while GLP-1 and GIP suppress appetite and improve insulin sensitivity. The triple combination produces a synergistic effect no single- or dual-agonist has matched in published trials.

Safety

Adverse events were primarily gastrointestinal (nausea, vomiting, diarrhoea), consistent with the GLP-1 class and most common during dose escalation. No serious dose-limiting toxicities were reported. A mean heart rate increase of approximately 5–7 bpm was observed at higher doses — a known class effect seen with other GLP-1 agonists.

Context in the GLP-1 Landscape

The Phase 2 data positioned retatrutide as the most efficacious weight-loss peptide in clinical development at the time of publication, surpassing tirzepatide's SURMOUNT-1 result of 22.5% and semaglutide's STEP-1 result of 14.9%. Phase 3 trials are ongoing as of 2026.