The Eli Lilly logo appears on the company’s office in San Diego, California, U.S., Nov. 21, 2025.
Mike Blake | Reuters
Eli Lilly on Thursday said its next-generation obesity drug delivered strong weight loss and reduced knee arthritis pain in a late-stage study, clearing the first of several upcoming trials on the weekly injection.
The highest dose of the drug helped patients with obesity and a type of knee arthritis lose an average of 23.7% of their body weight at 68 weeks, when analyzing all participants, including those who discontinued treatment.
When evaluating only patients who stayed on the drug – essentially the best-case scenario – the highest dose delivered 28.7% weight loss on average.
It is the first late-stage data on the retatrutide, which works differently from existing injections and appears to be more effective. Eli Lilly is betting big on retatrutide as the next pillar of its obesity portfolio after its weight loss injection Zepbound and its upcoming pill.
It’s a critical part of the drugmaker’s plan to maintain its market share majority over Novo Nordisk in the booming market for weight loss and diabetes drugs. Some analysts estimate the segment could be worth about $100 billion by the 2030s.
Retatrutide also met the trial’s other main goal of reducing pain from knee osteoarthritis – a common condition that wears down the joint’s cartilage and leads to pain and stiffness – by up to 62.6% on average when analyzing all patients, based on a widely used survey. More than one in eight patients who took the drug were completely free from knee pain by the end of the trial, Eli Lilly said.
The company believes retatrutide “could become an important option for patients with significant weight loss needs and certain complications, including knee osteoarthritis,” Kenneth Custer, president of Lilly Cardiometabolic Health, said in a statement.
But roughly 18% of patients on the highest dose of the drug stopped treatment due to side effects, compared to 4% of those in the placebo group. Eli Lilly said those dropout rates were “highly correlated” to patients’ starting BMI and included discontinuations due to “perceived excessive weight loss.”
Around 43% of patients on the highest dose experienced nausea, while roughly 33% and 20.9% had diarrhea and vomiting, respectively.
The study, called TRIUMPH-4, didn’t solely focus on weight loss, meaning that other trials specifically designed for that outcome could produce different or higher results. Eli Lilly expects to report findings from seven additional phase three trials on the drug by the end of 2026.
Dubbed the “triple G” drug, retatrutide works by mimicking three hunger-regulating hormones – GLP-1, GIP and glucagon – rather than just one or two like existing treatments. That appears to have more potent effects on a person’s appetite and satisfaction with food than other treatments.
Tirzepatide, the active ingredient in Eli Lilly’s weight loss shot Zepbound, mimics GLP-1 and GIP. Novo Nordisk’s obesity drug Wegovy mimics only GLP-1.
Higher doses of tirzepatide helped patients with obesity lose up to 22.5% of their body weight on average in late-stage studies.
As Eli Lilly establishes an edge in the space, its chief rival Novo Nordisk is racing to catch up. In March, Novo Nordisk said it agreed to pay up to $2 billion for the rights to an early experimental drug from the Chinese pharmaceutical company United Laboratories International.
Novo Nordisk’s newly acquired drug is a clear potential competitor to retatrutide because it similarly uses a three-pronged approach to promoting weight loss and regulating blood sugar. But Novo Nordisk’s treatment is much earlier in development, meaning it will take several years before it reaches patients.
