The drug, fluvoxamine, has been safely prescribed for nearly 30 years as a treatment for obsessive-compulsive disorder. But when the coronavirus started spreading, researchers were drawn to the medication because of its ability to reduce inflammation, potentially allowing it to quell the body’s overwhelming response to a coronavirus infection.

Several smaller studies of fluvoxamine earlier in the pandemic showed promising results, but none was as large or persuasive as the one published on Wednesday by a group of researchers in Canada, the United States and Brazil, outside scientists said. Among nearly 1,500 Covid patients in Brazil given either fluvoxamine or a placebo, the drug reduced the need for hospitalization or prolonged medical observation by one-third, the study found. It was published in The Lancet Global Health.

Some patients struggled to tolerate the drug and stopped taking it, the study said, raising a question among outside scientists about whether they had yet identified the ideal dose. But among those who had largely followed doctors’ orders, the benefits were even more striking. In those patients, the drug reduced the need for hospitalization by two-thirds and slashed the risk of dying: One Covid patient given fluvoxamine died, compared with 12 given a placebo.

“That’s really good,” said Dr. David Boulware, an infectious disease scientist at the University of Minnesota who worked on a smaller, real-world study of the drug in Covid patients in California. Plus, he added, “it’s not a shiny new, expensive drug. The nice thing about this is it has a known safety profile.”