In a major development in obesity treatment, Eli Lilly and Company has announced that its next-generation experimental weight-loss drug, retatrutide, has delivered striking results in a late-stage clinical trial. The Indiana-based pharmaceutical giant said the drug helped participants lose up to 85 pounds (around 30% of their body weight) in extended analysis, marking one of the most significant advances yet in obesity treatment.
A major finding of the study shows that participants receiving the highest dose of retatrutide achieved an average weight loss of 28.3% of their body weight over 80 weeks. This translated to about 70.3 pounds (31.9 kg) on average. Nearly half of the participants on the highest dose achieved at least 30% weight loss, a level often associated with outcomes seen in bariatric surgery, highlighting the drug’s strong metabolic impact.
What the TRIUMPH-1 trial revealed about weight loss outcomes
The findings come from Eli Lilly’s Phase 3 TRIUMPH-1 clinical trial, a large, randomized, double-blind, placebo-controlled study evaluating retatrutide in adults with obesity or overweight conditions who also had at least one weight-related health complication, but without diabetes.
The trial included 2,339 participants who were randomly assigned to receive either a placebo or one of three doses of retatrutide - 4 mg, 9 mg, or 12 mg - administered once weekly for 80 weeks. All doses were gradually escalated to help improve tolerance.
Results showed a clear dose-dependent response. Participants on the 4 mg dose lost an average of 47.2 pounds (19.0%). Those on 9 mg lost 64.4 pounds (25.9%), while those on the highest 12 mg dose achieved the strongest results, losing 70.3 pounds (28.3%) on average.
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Eli Lilly reported that 45.3% of participants on the 12 mg dose achieved at least 30% weight loss, while many others moved below clinical obesity thresholds. In a longer 104-week extension, weight loss reached up to 85 pounds (30.3%) in some participants with higher baseline BMI.
The company described retatrutide as a “first-in-class” triple hormone receptor agonist targeting GIP, GLP-1, and glucagon pathways together. These hormones play key roles in appetite regulation, insulin sensitivity, and energy metabolism, which may explain the strong weight-loss effects.
Along with weight reduction, participants also showed improvements in waist circumference, cholesterol levels, triglycerides, blood pressure, and other cardiometabolic risk markers.
“Importantly, treatment with retatrutide not only resulted in robust weight reduction, but also in clear improvements in assessed cardiometabolic health measures. For patients I see in clinic, retatrutide may potentially be a highly impactful future tool to treat their obesity and transform their health trajectory,” said Ania Jastreboff, M.D., Ph.D., Professor of Medicine & Pediatrics (Endocrinology) at the Yale School of Medicine.
Safety findings, side effects, and comparison with existing drugs
Eli Lilly also reported a range of side effects, consistent with other incretin-based weight-loss therapies.
The most common adverse effects were gastrointestinal. Nausea affected up to 42.4% of participants on the highest dose, compared with 14.8% in the placebo group. Diarrhea was reported in up to 34.1%, while constipation affected about one-quarter of participants. Vomiting was also more frequent at higher doses, affecting up to 25.3% of patients.
Upper respiratory infections and fatigue were also observed, though at similar levels in both treatment and placebo groups. A small proportion of participants discontinued treatment due to side effects, with discontinuation increasing at higher doses.
Despite this, most side effects were described as mild to moderate and generally manageable, with many resolving during the treatment period.
Retatrutide stands out because it targets three metabolic pathways instead of one or two, unlike earlier obesity drugs such as semaglutide or tirzepatide-based therapies. This triple mechanism is believed to enhance both appetite suppression and energy expenditure.
Eli Lilly also reported improvements in cardiometabolic markers, including HDL cholesterol, triglycerides, and blood pressure, suggesting broader health benefits beyond weight loss alone.
‘No other drug has come close’: Expert insights
Dr Rajiv Kovil, Head of Diabetology and Weight Loss Expert at Zandra Healthcare, said one of the most striking findings from the TRIUMPH-1 trial was not just the overall weight loss, but the proportion of participants achieving extremely high reductions in body weight.
“What is unique is that the highest dose achieved almost 28.3% weight loss over 80 weeks, but importantly, almost half of these patients lost more than 30% of their body weight,” he said.
According to Dr Kovil, retatrutide stands apart from earlier obesity drugs because it is a triple agonist targeting GLP-1, GIP, and glucagon receptors together, unlike older therapies that mainly act on one or two pathways.
He explained that another important aspect of the study was the absence of a clear “plateau effect” in weight reduction. With earlier drugs such as semaglutide and tirzepatide, weight loss often stabilises after a certain period, typically around 60 weeks. However, he noted that participants in the retatrutide trial appeared to continue losing weight even after prolonged treatment.
“Here, there is no plateauing. This is an 80-week study, and how much pharmacologically we can make people lose weight will only become clear once we start using it clinically,” he said.
Dr Kovil added that the findings suggest the drug may potentially approach outcomes usually associated with bariatric surgery.
“That kind of weight loss otherwise is usually seen only with gastric bypass surgery. No other drug has come close to it yet,” he said, adding that even the lowest 4 mg dose resulted in nearly 20% weight loss.
He described the therapy as “probably just the beginning of a new generation of engineered therapeutics in the metabolic segment.”
Speaking about safety concerns, Dr Kovil said the gastrointestinal side effects observed in the trial - including nausea, diarrhea, constipation, and vomiting - are largely similar to those already seen with GLP-1–based therapies currently on the market.
“All the GLP-1-related gastrointestinal side effects will always be there,” he said.
However, he noted that earlier studies had raised concerns about abnormal sensory symptoms linked to glucagon receptor activity, though the current study did not show major unexpected adverse effects.
“There was concern in earlier phases about some abnormal sensory symptoms in the feet and other areas with retatrutide, but this study has not really thrown that up,” he said.
Dr Kovil also pointed out that glucagon receptor agonists are still a relatively newer area in obesity medicine, meaning researchers are continuing to study whether any long-term side effects may emerge in the future.
At the same time, he said these therapies may offer additional benefits for liver disease. He noted that earlier data on retatrutide showed strong potential in resolving metabolic dysfunction-associated steatohepatitis (MASH), formerly known as fatty liver disease.
Discussing which patients may benefit most from the drug if approved in the future, Dr Kovil said retatrutide could become especially important for people with severe or morbid obesity who require very large reductions in body weight.
“There are patients who need 30% to 40% weight loss, and currently those patients are often advised lifestyle intervention followed by bariatric surgery,” he said.
Dr Kovil said the arrival of triple agonist therapies could eventually expand treatment choices for doctors. “Semaglutide may give you around 10–12% weight loss, tirzepatide around 20%, and this could give you up to 30%. We have far more options today for doctors to choose from,” he said.
This story is done in collaboration with First Check, which is the health journalism vertical of DataLEADS