Retatrutide, the experimental obesity drug Eli Lilly has been developing as a successor to its own tirzepatide (Zepbound/Mounjaro), has posted the most powerful weight-loss results of any obesity medicine reported to date. The full late-stage trial data were published Tuesday in the New England Journal of Medicine.
In the Phase 3 trial — run at 131 sites across 11 countries and launched in 2023 — 2,339 participants with obesity were split into four roughly equal groups receiving 4 mg, 9 mg or 12 mg of retatrutide, or a placebo. Average starting weight was about 113 kilograms (250 pounds) and average body-mass index was 40. At the highest dose, participants lost an average of 25% of their body weight at 80 weeks and 30% by an extension period at 104 weeks, with more than a third of all participants losing 30% or more. Lilly's own topline figures, which use a different statistical measure, put the 12 mg arm at 28.3% (70.3 pounds) at 80 weeks, with 45.3% of those participants crossing the 30% threshold.
The trial's wider health effects were just as striking. In a subset with obesity-linked knee osteoarthritis, knee pain fell by as much as 62%. Among participants with obesity-linked obstructive sleep apnea, the number of breathing events per hour dropped by up to 57%. Blood pressure, triglycerides and LDL cholesterol all improved, and among the more than one-third of participants who began the trial with prediabetes, the condition had resolved in more than 90% of them by the end.
The mechanism is a single synthetic peptide that mimics three hormones at once. Where semaglutide (Ozempic/Wegovy) targets GLP-1 and tirzepatide adds GIP, retatrutide adds glucagon as a third receptor target. It is far more potent than natural GIP at that receptor — roughly ninefold — and is stabilised to stay active in the blood for about six days, allowing a weekly injection.
Safety looked comparable to existing GLP-1 drugs: mostly transient, mild-to-moderate gastrointestinal symptoms such as nausea, diarrhoea and constipation. Some participants reported dizziness and low blood pressure, more often among those already on blood-pressure medication. Ten people had cardiovascular events and there were 10 reports of pancreatitis, one and two of them respectively in the placebo group — numbers too small to assess the risk of rare events. The trial's most significant limitation is that it did not compare retatrutide against tirzepatide, leaving the relative benefit unproven. The drug is not expected to reach the market before 2027.
