The U.S. Food and Drug Administration (FDA) has issued a new warning about the known side effects of hydroxychloroquine and chloroquine, two antimalarial drugs (also used in the treatment of chronic rheumatoid arthritis and lupus), on the same day that a group of researchers published a paper in the Journal of the American Medical Association about their termination of a study investigating the potential of chloroquine as a potential COVID-19 treatment.

The drugs can have dangerous side effects, and the study written about in the AMA journal was ended early because the “primary outcome” was the death of study participants, with 22 deaths resulting. Before it was abruptly ended, the researchers found a lethality rate of 39% in the group of its subjects who took high dosages, and 15% in the low-dosage group, with an overall lethality rate of 27%.

“Our study raises enough red flags to stop the use of a high-dosage regimen, because the risks of toxic effects overcame the benefits,” the researchers said in their findings.

The FDA’s alert on April 24 doesn’t make specific mention of this study, but the 81-person phase II clinical trial represents one of the largest to date. The FDA notice does advise that reports they have received of deaths in COVID-19 patients receiving hydroxychloroquine, either alone or in tandem with other drugs, including an antibiotic known as azithromycin (which all patients in the investigation published in the AMA journal were taking), can result in “abnormal heart rhythms such as QT interval prolongation, dangerously rapid heart rate called ventricular tachycardia and ventricular fibrillation, and in some cases, death.”

Hydroxychloroquine and chloroquine gained widespread attention because Donald Trump advocated them as likely effective treatments for COVID-19, despite a lack of significant scientific evidence or any clinical studies done about their safety, likely due to early, small-scale investigations that showed they might have some potential. He repeatedly touted the drugs as proven safe, since they have been cleared for use previously in treating other medical conditions, but didn’t appear to grasp that this does not mean they aren’t dangerous or perhaps deadly when taken in the context of other conditions, or for individuals with other risk characteristics.