What is ivermectin, and how did people get the idea it can treat covid?

Ivermectin tablets in Jakarta, Indonesia. The U.S. Food and Drug Administration has warned Americans against taking ivermectin, an anti-parasitic medication, to prevent or treat covid-19. (Dimas Ardian/Bloomberg)

In an earlier version of this story, David Boulware incorrectly cited an amount and concentration of animal ivermectin available online and calculated that it would be 100 times the normal human dosage, leading to toxicity. Ivermectin, at the amounts and concentration typically available online, would be more than seven times the normal dosage for an adult human, which could still lead to toxicity. This story has been corrected.

The idyllic photo of a chestnut horse appeared on the U.S. Food and Drug Administration’s Instagram feed in August, along with a blunt caption: “You are not a horse. Stop it with the #Ivermectin. It’s not authorized for treating #COVID.”

The post was one of several stark warnings issued about ivermectin — an anti-parasitic medication being promoted by prominent conservative media figures and politicians, as well as some physicians, as an effective treatment for covid-19, despite the lack of scientific evidence showing there are benefits of taking the drug for that purpose.

Doctors dismayed by patients who fear coronavirus vaccines but clamor for unproven ivermectin

Interest in ivermectin has surged during this summer’s rapid rise in coronavirus infections fueled by the highly transmissible delta variant. Prescriptions have soared, and some people have resorted to taking forms of the anti-parasitic intended for large animals, leading to spikes in calls to poison control centers around the country.

Elevated by social media and right-wing pundits and politicians, the drug is not recommended by either the FDA or its manufacturer to treat covid-19. (Video: Joshua Carroll/The Washington Post)

For many experts, the ivermectin craze is stirring feelings of pandemic deja vu.

“We’ve already had the sad example of hydroxychloroquine, which is used for malaria, which clearly did not show any positive effects,” said Sunil Parikh, an associate professor of epidemiology and infectious diseases at the Yale School of Public Health, referring to the drug pushed by President Donald Trump and others during the earlier stages of the pandemic.

“I’m afraid at the moment we’re heading down that way with ivermectin,” Parikh said.