The anti-parasitic drug ivermectin, which has surged in popularity as an alternative treatment for Covid-19 despite a lack of strong research to back it up, showed no sign of alleviating the disease, according to results of a large clinical trial published on Wednesday.

The study, which compared more than 1,300 people infected with the coronavirus in Brazil who received either ivermectin or a placebo, effectively ruled out the drug as a treatment for Covid, the study’s authors said.

The researchers shared a summary of these results in August during an online presentation hosted by the National Institutes of Health, but the full data set had not been published until now, in The New England Journal of Medicine.

“Now that people can dive into the details and the data, hopefully that will steer the majority of doctors away from ivermectin towards other therapies,” Dr. David Boulware, an infectious-disease expert at the University of Minnesota, said.

For decades, ivermectin has been widely used to treat parasitic infections. Early in the pandemic, when researchers were trying thousands of old drugs against Covid-19, laboratory experiments on cells suggested that ivermectin might block the coronavirus — though at much higher concentrations than would be safe for human use.

Some small studies suggested possible benefits in humans, but subsequent analysis found the studies to be flawed and the benefits illusory. The clinical trial whose data was published on Wednesday was much larger and more rigorous.

Researchers in Brazil provided the drug to 679 patients over the course of three days between March and August 2021 in a double-blinded treatment, meaning that neither the patients nor the medical staff knew whether any particular patient was receiving a Covid treatment drug or a placebo.

The results were clear: Taking ivermectin did not reduce a Covid patient’s risk of ending up in the hospital.

The researchers zeroed in on different groups of volunteers to see if they experienced benefits that others didn’t. For example, it might have been possible that ivermectin worked only if taken early in an infection. But volunteers who took ivermectin in the first three days after a positive coronavirus test turned out to have worse outcomes than did those in the placebo group.

There are other large randomized trials of ivermectin, with thousands of volunteers, that are still in progress and have yet to share their results. The National Center for Advancing Translational Sciences, which is part of the N.I.H., has been running one closely watched trial of ivermectin and several other drugs for Covid patients for more than a year, with no results released yet.