Lilly stock slips after study finds patients who stop Zepbound regain weight

People who stopped taking Eli Lilly and Co.’s blockbuster drug Zepbound after about eight months regained half the weight they’d lost a year later, yet were significantly thinner than when they had started the obesity drug, according to a new study.

Taking Zepbound for 36 weeks resulted in a 20.9% loss of body weight, according to the study published Monday in the Journal of the American Medical Association. But after 88 weeks, a group no longer taking the drug after the first eight months had still lost 9.9% of their weight. Those who stayed on the drug the entire time lost an average 25.3%.

Lilly shares fell 4% in trading Monday after the study’s release, to $574.33 each.

Lilly’s tirzepatide is sold under brand name Zepbound for weight loss and has been available as Mounjaro for type 2 diabetes. The drugs produced sales of $1.41 billion in the third quarter.

Earlier studies have shown that people who stop taking popular obesity drugs from Lilly and Novo Nordisk A/S called GLP-1s are prone to regaining weight. Still, the results indicate that some benefits are retained a full year later, said Louis Aronne, an obesity expert at Weill Cornell Medicine and lead study author.

“I was surprised how good the result was,” Aronne said in an interview. He’s a consultant for both Lilly and Novo, which makes the competing weight-loss drug Wegovy. The study clearly shows the need for continual use of these drugs for optimal benefit, Aronne said.

“Patients, providers and the public do not always understand obesity is a chronic disease that often requires ongoing treatment,” Jeff Emmick, Lilly’s senior vice president of product development, said in a statement.

The researchers gave Zepbound for 36 weeks to people with obesity or who were overweight. After that, half the patients were randomly assigned to stay on it for another year while the other half got a placebo. Those who stayed on Zepbound for the full 88 weeks saw their weight loss plateau around the 64-week mark.

Lilly had revealed some of the results of the study in July, but this is the first time the full details have been published.

A study done last year by Novo Nordisk of semaglutide, which is used in its rival weight-loss drug Wegovy, found that patients regained two-thirds of lost body weight after discontinuing the therapy.