A flash of potential good news from the front lines of the coronavirus pandemic: A treatment is showing promise. Doctors in France, South Korea and the U.S. are using an antimalarial drug known as hydroxychloroquine with success. We are physicians treating patients with Covid-19, and the therapy appears to be making a difference. It isn’t a silver bullet, but if deployed quickly and strategically the drug could potentially help bend the pandemic’s “hockey stick” curve.

Hydroxychloroquine is a common generic drug used to treat...