Since Covid-19 patients started showing up at clinics and hospitals a year and a half ago, doctors and researchers have been hard at work trying to figure out how to treat them. Most drugs and treatments haven’t panned out, producing either no results or small ones in large-scale clinical trials. Many of the few that work are expensive and difficult to administer.

Hydroxychloroquine, enthusiastically endorsed by President Trump last year, has been shown to have no measurable benefits. New drugs like monoclonal antibodies — proteins meant to imitate the immune system’s response to the disease — have been approved by regulators but must be administered by a doctor through an IV or series of injections.

But scientists haven’t stopped searching, and the results of a new massive clinical trial suggest they’re getting somewhere. In a large, randomized clinical trial conducted with thousands of patients over the past six months, researchers at McMaster University tested eight different Covid-19 treatments against a control group to figure out what works.

One drug stood out: fluvoxamine, an antidepressant that the Food and Drug Administration has already found to be safe and that’s cheap to produce as a generic drug.