Both chloroquine and hydroxychloroquine interfere with ventricular repolarization, leading to prolongation of the QTc interval and an increased risk of torsades de pointes (TdP). This effect is dependent on dose: studies involving volunteers found mean increases in QTc of 6.1 ms after a dose of 600 mg and 28 ms after a dose of 1200 mg. 13, 14 However, the effect varies among individuals and can be pronounced. Among 30 children given short courses of chloroquine for malaria, 1 experienced an increase in the QTc interval of 64 ms after just 1 day of treatment. 15 Azithromycin itself does not usually cause clinically significant prolongation of the QTc interval, 16 but its use in combination with either chloroquine or hydroxychloroquine could theoretically increase the risk of TdP. Reassuringly, an animal model found no evidence of such an interaction, 17 and the combination has been used safely in patients with malaria. 18, 19 Nevertheless, given limited experience in patients with COVID-19 and the potential for use of these drugs in patients with cardiac disease or those taking other drugs that delay repolarization, monitoring of the QTc interval at baseline and daily for the duration of treatment is advised, especially if azithromycin is coprescribed. Daily monitoring is impractical during prophylactic treatment, but assessment of the QTc interval at baseline is advised, especially for individuals with cardiac disease. It is prudent to correct electrolyte disorders and, where possible, avoid or minimize use of other drugs known to prolong the QT interval (Box 2).