Researchers in Brazil were testing chloroquine in 81 hospitalized COVID-19 patients. About half were prescribed 450 mg twice daily for five days, and the others were prescribed 600 mg for 10 days, The New York Times reported. But within three days, some patients taking the higher dose developed serious heart rhythm disorders, resulting in 11 deaths. The researchers immediately stopped the higher-dose part of the trial. The trial did not have enough patients in the lower-dose trial to conclude whether chloroquine was effective in patients with severe cases of COVID-19.

“To me, this study conveys one useful piece of information, which is that chloroquine causes a dose-dependent increase in an abnormality in the electrocardiogram that could predispose people to sudden cardiac death,” David Juurlink, M.D., an internist and the head of the division of clinical pharmacology at the University of Toronto, told The Times.

Patients in the trial also received the antibiotic azithromycin, which also has heart rhythm disorder risks. Hospitals in the United States are using azithromycin to treat COVID-19 patients, often in combination with hydroxychloroquine, according to The Times.