Study summary
Retatrutide Phase 2 NEJM summary
Close reading of the Jastreboff et al. (NEJM 2023) Phase 2 retatrutide weight-loss trial — design, dose-response data, side-effect signals and methodological caveats.
Trial design
Jastreboff et al. (New England Journal of Medicine, 2023; doi:10.1056/NEJMoa2301972) reported a Phase 2, double-blind, randomised, placebo-controlled trial of retatrutide in adults with BMI ≥30 (or ≥27 with weight-related comorbidities). 338 participants were randomised across placebo and retatrutide arms at 1, 4, 8 and 12 mg weekly, with a 48-week treatment period and an extended dose-titration schedule.
Primary outcome
Mean percentage change in body weight at 48 weeks was the primary endpoint. Placebo-adjusted reductions were: -7.2% (1 mg), -12.9% (4 mg), -17.3% (8 mg) and -17.5% (12 mg). The total observed (not placebo-adjusted) reductions were approximately -24% at the 12 mg dose — a magnitude that exceeded what tirzepatide had achieved in SURMOUNT-1 at 15 mg over a longer 72-week period.
Secondary metabolic effects
HbA1c reductions in participants with prediabetes (about 38% of the cohort) mirrored the body-weight response: dose-dependent reductions of 0.4 to 1.0% across the active arms. Lipid panel improvements (LDL-C and triglyceride reductions) and blood-pressure decreases (systolic 4-8 mmHg) accompanied the weight loss.
Safety signals
Gastrointestinal side effects (nausea, diarrhoea, vomiting, constipation) were the dominant adverse-event class, consistent with the wider GLP-1 / dual-agonist experience. Two retatrutide-specific signals emerged: a small dose-dependent increase in resting heart rate (3-6 bpm in the 8 mg and 12 mg arms) and modest reductions in HDL-C at higher doses. Whether these signals persist or reverse across longer Phase 3 exposure is one of the central questions for the TRIUMPH programme.
Methodological caveats
The Phase 2 sample size (338) and 48-week duration are smaller and shorter than the tirzepatide Phase 3 SURMOUNT comparators. Cross-trial comparison should be done cautiously; only when Phase 3 retatrutide data is fully reported will direct benchmarking be defensible.
See full profiles: Retatrutide · Tirzepatide · the head-to-head comparison.