https://arstechnica.com/science/2020/03/fda-approves-the-emergency-use-of-chloroquine-for-covid-19/

On Saturday, the Food and Drug Administration issued an Emergency Use Authorization that will allow patients suffering from COVID-19 to be treated using drugs without clear evidence of the drugs' efficacy. The move comes after President Donald Trump has touted the drugs' potential several times on the basis of tiny, anecdotal trials. There have also been reports of hoarding of the drugs, which are needed by people with some autoimmune disorders.

The drugs in question are relatives of chloroquine, specifically chloroquine phosphate and hydroxychloroquine sulfate. Originally developed as an antimalarial, the drug has a variety of effects, including the ability to reduce immune activity. That has made it useful for the treatment of autoimmune disorders such as lupus and rheumatoid arthritis. Given its multiple effects, it's not surprising that the drug also has a variety of side effects, the most significant probably being a slowing of the heart's rhythm that can potentially lead to fatal complications. (Technically, the drug extends the QT interval.)