President Trump tweeted about it enthusiastically.
But the study had multiple errors, flaws and biases, according to letters to the journal’s editors.
The study was conducted at the Henry Ford Health System in Detroit. It contradicts many other studies, which have found the drug does not help Covid-19 patients.
“We’ve acknowledged the varying conclusions multiple studies have reached, along with the limitations of our retrospective [hydroxychloroquine] study as well as those of other published studies on the topic,” according to a statement on Wednesday from Tammy Battaglia, a spokeswoman for Henry Ford.
Henry Ford doctors chose to prescribe hydroxychloroquine to some patients, but not to others — and that may have introduced some bias into the study, the letters suggested.
For example, the patients in the Henry Ford study who were given hydroxychloroquine had fewer risk factors for heart disease, the Albany researchers, Eli Rosenberg, David Holtgrave and Tomoko Udo, wrote in their letter.
Also, the hydroxychloroquine patients were more than twice as likely to be given steroids, a treatment known to be effective against Covid-19.
The Detroit study was not a randomized clinical trial, which is considered the gold standard in medicine and helps avoid these potential biases. In such trials, patients are randomly assigned to take a drug or not take it, which means the two groups should be very similar.
In their study, Henry Ford doctors emphasized that hydroxychloroquine worked for their patients because it was prescribed very early in their hospitalization. They said doctors in the University at Albany study prescribed the drug too late.
But in their letter to the editor, the Albany researchers said the doctors in their study prescribed hydroxychloroquine at about the same point during the course of hospitalization as the patients in Detroit.
The Albany researchers said the Henry Ford doctors “make multiple statements that are not factually aligned with our published research.”
CNN’s Dana Vigue contributed to this report.