The study, published Friday in the medical journal the Lancet, is the largest analysis to date of the risks and benefits of treating covid-19 patients with antimalarial drugs. It is based on a retrospective analysis of medical records, not a controlled study in which patients are divided randomly into treatment groups — a method considered the gold standard of medicine. But the sheer size of the study was convincing to some scientists.

The new analysis — by Mandeep Mehra, a Harvard Medical School professor and physician at Brigham and Women’s Hospital, and colleagues at other institutions — included patients with a positive laboratory test for covid-19 who were hospitalized between Dec. 20, 2019, and April 14, 2020, at 671 medical centers worldwide. The mean age was 54 years, and 53 percent were men. Those who were on mechanical ventilators or who received remdesivir, an antiviral drug made by Gilead Sciences that has shown promise in decreasing recovery times, were excluded.

The new study’s findings cannot necessarily be extrapolated to people with mild illness at home or those, like Trump, who are taking the antimalarials as a prophylactic. The president stunned many doctors earlier this week when he said he was taking a pill “every day” — despite FDA warnings that the use of the drug should be limited to those in a hospital setting or in clinical trials. (He has since said he is close to finishing his course of treatment and would stop taking the medication in “a day or two.”)

There have been at least 13 studies in recent months on hydroxychloroquine or chloroquine as a treatment for covid-19 patients. They have included randomized controlled studies and observational analyses encompassing patients on the continuum from mild illness to those near death. Evidence of any benefit, such as viral clearance or improved symptoms, has been almost nonexistent. But many found an increased risk in adverse cardiac reactions — especially when combined with the antibiotic azithromycin.

Earlier this month, some proponents of hydroxychloroquine seized on a study out of New York University’s Langone Health center that threw zinc into the mix with hydroxychloroquine and azithromycin, and showed the treated group had a higher rate of survival. But researchers emphasized that it only showed that the combination had some promise. They said the results also could have been due to other factors, such as the zinc being added to patients’ regimens later in the pandemic when hospital treatments and procedures had been refined.

Geoffrey Barnes, a cardiovascular specialist at the University of Michigan, said the study’s approach and its findings were “striking” in making the case that “the risk with these drugs is real.” However, he said that due to the enthusiasm some Americans have for the drug and the Lancet study’s findings, randomized trials are even more important.

When the first large wave of sick patients began showing up at hospitals in March, doctors had very little to offer them. As a result, many took a gamble on hydroxychloroquine. The drug had been shown to have strong antiviral properties in cell cultures, was widely available and was thought to be rather benign in terms of side effects. For years, hydroxychloroquine has been considered a generally safe and effective treatment for malaria, lupus and rheumatoid arthritis.

But those findings of safety were at lower doses than were being used at hospitals during the early days of the surge in patients in the United States and mostly in patients who were healthy. The population infected with covid-19 in hospitals, it turned out, was already at higher risk of cardiovascular complications because many suffer from high blood pressure or other heart issues. Doctors also discovered that, to their surprise, the novel coronavirus appeared to directly or indirectly attack the heart, including by reducing its ability to pump, creating an imbalance in its electrical rhythms, and attacking blood vessels.

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