Ivermectin was popularized as an alternative COVID-19 treatment despite a lack of strong evidence it helps. The recent study is among the largest that has failed to show a benefit.
In the study, researchers in Brazil compared more than 1,300 patients, some given ivermectin and others given a placebo, and found no difference between the groups.
"In this randomized trial, the administration of ivermectin did not result in a lower incidence of medical admission to a hospital or prolonged emergency department observation for Covid-19 among outpatients at high risk for serious illness," researchers said.
Early in the pandemic, lab experiments on cells suggested ivermectin could have some promise, but studies in people failed to back that up.
Some studies on ivermectin are ongoing, but today we have several highly effective vaccines and COVID-19 treatments, with robust studies in people showing they work to dramatically reduce COVID-19 risk.
Major health institutions, including the Food and Drug Administration and the Centers for Disease Control and Prevention, have issued warnings urging against the use of ivermectin as a COVID-19 treatment.
"Let me just say very clearly that ivermectin is not a recommended treatment for COVID-19," U.S. Surgeon General Vivek Murthy said last August. "It is not a recommended drug to prevent COVID-19."
The drug, commonly used for livestock and pets in an animal-grade formula, is also FDA-approved in a human formula to treat parasitic infections and skin conditions such as scabies.
Use of ivermectin surged during the pandemic; internal data from the CDC reviewed by ABC News estimated a 19-fold increase of the drug being distributed during the first week of August 2021, amid the delta surge.
Online telehealth services have helped provide easy access to the deworming drug.
Some studies cited by ivermectin advocates as showing benefits of the drug in combating COVID-19 have been retracted for flawed or fabricated data and analysis, while many randomized trials have shown no benefits.