Scientists in Brazil have stopped part of a study of a malaria drug touted as a possible coronavirus treatment after heart rhythm problems developed in one-quarter of people given the higher of two doses being tested.

Chloroquine and a newer, similar drug called hydroxychloroquine, have been pushed by President Donald Trump after some very small, early tests suggested the drugs might curb the virus from entering cells. But the drugs have long been known to have potentially serious side effects, including altering the heartbeat in a way that could lead to sudden death.