OLYMPIA—The Washington State Department of Health (DOH) warns against using Ivermectin, an antiparasitic drug commonly used for humans and animals, to treat or prevent COVID-19.

Ivermectin, first discovered in 1975 and commercialized in 1980, began making COVID-19 headlines last year, after Australian researchers reported that the drug could inhibit in vitro coronavirus replication in large doses. The drug has been permitted as treatment in some of the worst-hits areas of the world, including Slovakia, the Czech Republic, and various areas in Latin America.

The antiparasitic is approved...