“There was no indication that ivermectin is clinically useful,” said Edward Mills, one of the study’s lead researchers and a professor of health sciences at Canada’s McMaster University in Hamilton, Ontario. Dr. Mills on Friday plans to present the findings, which have been accepted for publication in a major peer-reviewed medical journal, at a public forum sponsored by the National Institutes of Health.

Dr. Mills and his colleagues looked at 1,358 adults who visited one of 12 clinics in the Minas Gerais region of Brazil with Covid-19 symptoms. The patients all had a positive rapid test for SARS-CoV-2, and were at risk of having a severe case for reasons including a history of diabetes, hypertension, cardiovascular disease or lung disease.

The researchers prescribed half of the patients a course of ivermectin pills for three days. The other half received a placebo. They tracked whether the patients were hospitalized within 28 days. The researchers also looked at whether patients on ivermectin cleared the virus from their bodies faster than those who received a placebo, whether their symptoms resolved sooner, whether they were in the hospital or on ventilators for less time and whether there was any difference in the death rates for the two groups.