Here is what we know about the drug:

What is hydroxychloroquine?

Hydroxychloroquine is a prescription medicine that was approved decades ago to treat malaria. It is also used to treat autoimmune diseases like rheumatoid arthritis and lupus. It is sometimes referred to by its brand name, Plaquenil, and is closely related to chloroquine, which is also used to treat malaria.

Why has hydroxychloroquine been considered as a possible treatment for the coronavirus?

There are several reasons. A promising laboratory study, with cultured cells, found that chloroquine could block the coronavirus from invading cells, which it must do to replicate and cause illness. However, drugs that conquer viruses in test tubes or petri dishes do not always work in the human body, and studies of hydroxychloroquine have found that it failed to prevent or treat influenza and other viral illnesses.

Reports from doctors in China and France have said that hydroxychloroquine, sometimes combined with the antibiotic azithromycin, seemed to help patients. But those studies were small and did not use proper control groups — patients carefully selected to match those in the experimental group but who are not given the drug being tested. Research involving few patients and no controls cannot determine whether a drug works. And the French study has since been discredited: The scientific group that oversees the journal where it was published said the study did not meet its standards.

A study from China did include a control group and suggested that hydroxychloroquine might help patients with mild cases of Covid-19, the disease caused by the coronavirus. But that study had limitations: It was also small, with a total of 62 patients, and they were given various other drugs as well as hydroxychloroquine. The doctors evaluating the results knew which patients were being treated, and that information could have influenced their judgment. Even if the findings hold up, they will apply only to people who are mildly ill. And the researchers themselves said more studies were needed.

Another reason the drug has been considered for coronavirus patients is that it can rein in an overactive immune system, which is why it is used to treat lupus and rheumatoid arthritis. In some severe cases of Covid-19, the immune system seems to go into overdrive and cause inflammation that can damage the lungs and other organs. Doctors hope hydroxychloroquine might calm the condition, sometimes called a cytokine storm, but so far there is no proof that it has that effect.