The drug has generated excitement because a laboratory study, with cultured cells, found that chloroquine could block the coronavirus from invading cells, which it must do to replicate and cause illness. But drugs that show promise in the laboratory do not always translate to success in the human body, and other studies have found that it failed to prevent or treat influenza and other viral illnesses.

Early reports from doctors in China and France have said that hydroxychloroquine, sometimes combined with the antibiotic azithromycin, seemed to help patients. But the studies were small and did not use proper control groups — patients carefully selected to match those in the experimental group but who are not given the drug being tested. Research involving few patients and no controls cannot determine whether a drug works.

At most hospitals in the Ochsner Health system in Louisiana, including those in New Orleans, infected patients are routinely given a course of hydroxychloroquine. Patients in the intensive care unit are also given the drug if they have not received it earlier in their illness.

Dr. Leo Seoane, the chief academic officer at Ochsner Health, said the health system had declined to participate in research trials that included a placebo arm, in which some patients would be selected not to receive the drug. “We didn’t think it was ethical at this point in the crisis to withhold the therapies that could be beneficial,” he said.

But he acknowledged that even though the hospital gives the drug to nearly everyone who is admitted, the percentage of people who end up in the intensive care unit — about a third of those admitted — is similar to reports in other places where the drug is not used. “From a gut feeling, it’s hard for me to know that it is having an impact,” said Dr. Seoane, who is also a pulmonologist and critical care physician.