For the study published in JAMA Internal Medicine, researchers at the University of Pennsylvania focused on 125 health care workers. Some of them received hydroxychloroquine for eight weeks from April to July, while others got a placebo. Throughout that time, four of the 64 workers who got the drug ended up with COVID-19, while four of the 61 who got the placebo did as well. Six of those who tested positive developed coronavirus symptoms, but none needed to be hospitalized. As a result, the researchers said they “cannot recommend the routine use of hydroxychloroquine” to prevent infections among health care workers.

In June, a clinical trial published in the New England Journal of Medicine also showed hydroxychloroquine wasn’t effective in preventing coronavirus infections after exposure to the virus. The FDA has since removed its emergency use authorization for the drug as a coronavirus treatment, and in July it released a study showing how the drug could cause serious side effects in hospitalized patients.