(Bloomberg) -- People who stopped taking Eli Lilly & Co.’s 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 study.

Taking Zepbound for 36 weeks resulted in a 21% 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 8 months had still lost 10% of their weight. Those who stayed on the drug the entire time lost an average of 25%.

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. 

Lilly shares fell as much as 5.1% at 11:40 a.m. in New York trading Monday, their biggest drop in more than a month, before paring some of the decline.

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“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 the drug 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.

Read More: More Time on Mounjaro Meant More Weight Loss in Lilly Study

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

--With assistance from Madison Muller.

(Updates with shares in fourth paragraph.)

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