There has been a "rapid increase" in prescriptions of ivermectin among people looking to treat or prevent COVID-19 cases, according to a health alert issued Thursday by the Centers for Disease Control and Prevention. Ivermectin is an approved treatment for parasitic worms in animals and some rare conditions in humans, but not COVID-19 infections. "Adverse effects associated with ivermectin misuse and overdose are increasing, as shown by a rise in calls to poison control centers reporting overdoses and more people experiencing adverse effects," the CDC said Thursday. The Food and Drug Administration earlier this week asked people to stop taking the drug, which is not proven to treat COVID-19. "You are not a horse," the regulator said in a tweet that has since gone viral. The drug is one of several therapies including hydroxychloroquine that are not proven to treat COVID-19 in clinical studies but have become a worrisome issue for public-health experts as people seek alternative ways to address COVID-19 infections.