The Food and Drug Administration warned Friday that people should not take chloroquine and hydroxychloroquine to treat covid-19 outside of a hospital or formal clinical trial, citing reports of “serious heart rhythm problems.”

Many of those adverse effects occurred in patients with the virus who were treated with the anti-malaria drugs, often in combination with azithromycin, also known as Z-Pak. President Trump has described such drugs as a potential “game-changer” although results from clinical trials are not yet in to show whether they are effective.

“We will continue to investigate risks associated with the use of hydroxychloroquine and chloroquine for COVID-19 and communicate publicly when we have more information,” the FDA wrote.