Ivermectin, a controversial anti-parasitic drug that has been touted as a potential Covid-19 treatment, doesn’t speed recovery in people with mild illnesses. This is the result of a randomized controlled trial published in the journal JAMA on Thursday.

Ivermectin is typically used to treat parasitic worms in both humans and animals, but the scientific evidence of its effectiveness against the coronavirus is thin. Some studies have shown that the drug can prevent several different viruses from replicating in cells. And last year, researchers in Australia found that high doses of ivermectin suppressed SARS-CoV-2, the virus that causes Covid-19, in cell cultures.

Such findings had driven the use of the drug against Covid-19, especially in Latin America.

“Ivermectin is currently used extensively,” said Dr. Eduardo López-Medina, doctor and researcher at the Center for Pediatric Infectious Diseases in Cali, Colombia, who led the new study. “In many countries in the Americas and other parts of the world, this is part of national guidelines for treating Covid.”

But the drug has also been shown to be divisive. While some scientists see potential, others suspect that effective inhibition of the coronavirus may require extremely high, potentially unsafe doses. Health officials have also feared that people desperate for coronavirus treatments might be taking versions of the drug formulated for pets. (It’s often used to prevent heartworms in dogs.)

“There have been many conflicting views on these, sometimes extremely conflicting views,” said Dr. Carlos Chaccour, a researcher at the Barcelona Institute for Global Health who was not involved in the new study. “I think it’s become another hydroxychloroquine.”

But neither the proponents nor the critics had much rigorous data to support their views. There are few well-controlled studies on the drug’s effectiveness against Covid-19, although more are expected in the coming months. The National Institutes of Health treatment guidelines indicate that there is insufficient evidence to recommend “for or against” the use of the drug in Covid-19 patients.

Updated

March 4, 2021, 6:07 p.m. ET

In the new study, Dr. López-Medina and his colleagues happened to add more than 400 people who had recently developed mild Covid-19 symptoms to receive five-day treatment with ivermectin or a placebo. They found that Covid-19 symptoms lasted an average of about 10 days in people who received the drug, compared to 12 days in those who received the placebo, a statistically insignificant difference.

The new study adds much-needed clinical data to the debate over the drug’s use to treat Covid-19, said Dr. Regina Rabinovich, a global health researcher at the TH Chan School of Public Health at Harvard who was not involved in the study.

However, she noted that the study was relatively small and didn’t answer the most pressing clinical question of whether ivermectin can prevent serious illness or death. “Duration of symptoms may not be the most important clinical or health parameter,” she said.

The researchers found that seven patients in the placebo group got worse after entering the study compared to four in the ivermectin group, but the numbers were too small to draw any meaningful conclusion.

“There was a little signal there and it would be interesting to see whether or not this signal we saw is real,” said Dr. López Medina. “But that would have to be answered in a larger process.”

Dr. López-Medina also pointed out that the study population was relatively young and healthy, with an average age of 37 and few underlying diseases that can make Covid-19 more dangerous.

Larger studies currently in progress could provide more definitive answers, said Dr. Rabinovich, who stated that she was “completely neutral” about the potential benefits of ivermectin. “I only want data because there is such a mess in the field.”