The US Food and Drug Administration on Friday warned doctors against prescribing a malaria drug touted by President Donald Trump for treating the new coronavirus except in hospitals and research studies, AP reports.

In an alert, regulators flagged reports of sometimes fatal heart side effects among coronavirus patients taking hydroxychloroquine or the related drug chloroquine. The decades-old drugs, also prescribed for lupus and rheumatoid arthritis, can cause a number of side effects, including heart rhythm problems, severely low blood pressure and muscle or nerve damage.

The warning comes as doctors at a New York hospital published a report that heart rhythm abnormalities developed in most of 84 coronavirus patients treated with hydroxychloroquine and the antibiotic azithromycin, a combo Trump has promoted.