Doctors and hospitals are turning to decades-old antimalarial drugs to treat patients infected with the disease caused by the new coronavirus, as they work to repurpose existing therapies in a race to find effective treatments.

Antimalarial drugs chloroquine phosphate and hydroxychloroquine have both shown early signs of improving symptoms of some patients diagnosed with Covid-19, the respiratory disease caused by the coronavirus, based on reports by doctors and researchers in South Korea, France and China. Physicians in the...