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All doses of obesity drug retatrutide resulted in clinically meaningful weight loss, while people with severe obesity on the highest dose lost up to 30% of their body weight over two years, topline results of a phase III clinical trial have shown.
In an announcement published on 21 May 2026, Eli Lilly shared the results of the TRIUMPH-1 study, which involved 2,339 participants who were randomised to receive either retatrutide 4mg, 9mg, 12mg (building up every four weeks from 2mg), or placebo.
At 80 weeks, participants on 4mg of retatrutide lost an average of 17.6% of their body weight, compared with 23.7% on 9mg, 25.0% on 12mg and 3.9% on placebo, the study results revealed.
The researchers also observed that 532 participants with BMI ≥35 at week 0, who had tolerated their assigned dose of medication, were then escalated to the maximum tolerated dose (9mg or 12mg) for a further 24 weeks. At 104 weeks, these participants had achieved up to 30% body weight reduction, the researchers found.
Obesity drug retatrutide targets glucose-dependent insulinotropic polypeptide receptors, glucagon-like peptide-1 receptors and glucagon receptors. The drug is currently an investigational molecule and does not have regulatory approval.
In comparison, the highest available doses of tirzepatide (Mounjaro; Eli Lilly) and semaglutide (Wegovy; Novo Nordisk), have both demonstrated around 20% weight loss after 72 weeks in previous clinical trials (SURMOUNT-1; STEP UP).
Lead investigator Ania Jastreboff, professor of medicine and paediatrics at the Yale School of Medicine, said that “clear improvements in assessed cardiometabolic health measures” were also recorded during the study.
Commenting on the study, Hannah Beba, clinical lead for obesity at the West Yorkshire Health and Care Partnership, called the retatrutide topline results “genuinely striking”, noting that the 30% weight loss was “a level long associated with bariatric surgery”.
“Encouragingly, even the lower 4mg dose delivered an average 19.0% weight loss with fewer discontinuations due to adverse events, which matters for real-world tolerability,” she added.
Common adverse effects reported by participants who took the highest dose of retatrutide in the TRIUMPH-1 trial included nausea (42.4% of participants), diarrhoea (32.0% of participants), constipation (26.1% of participants) and vomiting (25.3% of participants).
The study results also showed that dysesthesia and urinary tract infections were reported in around one in ten patients on the highest doses of retatrutide but they were generally mild-to-moderate symptoms and the majority were resolved during treatment, while most participants continued taking retatrutide.
Discontinuation rates owing to adverse events were 4.1%, 6.9%, 11.3%, with retatrutide 4mg, 9mg, and 12mg, respectively, compared with 4.9% with placebo, the study results revealed.
“That said, retatrutide remains investigational and unlicensed, and the full safety data deserve careful scrutiny once peer-reviewed. If approved, its real impact will depend on pairing the medicine with sustained dietary, behavioural and pharmacist-led support,” Beba added.
Sehar Shahid, NPA board member for Scotland, also described the results as “striking”, adding that the results demonstrated “outcomes that set a new benchmark for the field”.
“However, retatrutide remains investigational and is not yet approved in the UK or United States, with regulatory submission still pending and approval not anticipated before 2027 at the earliest. As an obesity specialist pharmacist, I look forward to its eventual approval and the opportunity it will bring to support many more patients who haven’t yet found the right treatment for them,” she said.
Initial findings have already been shared from two previous retatrutide studies. Results from the TRIUMPH-4 trial, published in December 2025, showed participants with obesity and knee osteoarthritis taking retatrutide 12mg lost an average of 28.7% of their body weight at 68 weeks.
The results of the TRANSCEND-T2D-1 trial, published in March 2026, showed that participants with type 2 diabetes mellitus, who took retatrutide 12mg, lost an average of 16.8% of their body weight at 40 weeks.
Additional findings are due to be published in peer-reviewed journals and presented at the 86th annual American Diabetes Association Scientific Sessions, held in June 2026 in New Orleans, Louisiana, United States.
