The malaria drug hydroxychloroquine, which has been promoted as a potential treatment for Covid-19, is known to have potentially serious effects on heart rhythms. Now, corresponding author Flavio Fenton and a team of researchers have used an optical mapping system to observe exactly how the drug creates serious disturbances in the electrical signals that govern heartbeat. Related coverage: Research Horizons, ScienceDaily, Futurity, The Doctor's Channel, Cardiac Rhythm News.