Hydroxychloroquine is an effective drug for the treatment of diseases like lupus and malaria and because it is used off-label for maladies that act in a biologically similar way to how COVID-19, caused by the SARS-CoV-2 virus, acts in the body, there has been discussion that clinical trials might show it having a positive therapeutic impact.
Could it prevent COVID-19, though?
A clinical trial of health care workers finds that it does not, though the good news is that because the trial was health care workers they had low rates of infection anyway, likely due to other prevention measures. Masks, hand-washing, and social distancing work.