The U.S. Food and Drug Administration issued a strong and unusual warning on Saturday: “You are not a horse. You are not a cow. Seriously, y’all. Stop it.”

On Friday, Mississippi’s health department issued a warning that more than 70% of recent calls to the state’s poison center came after people took ivermectin bought at livestock supply centers.

The FDA was reacting to alarms from Mississippi, the state with the worst outbreak in the U.S., that people have been taking ivermectin to treat or prevent COVID-19. The drug is often used against parasites in livestock. Thomas Dobbs, the state health officer, said earlier this week he knew of only one hospitalization but was hearing reports of people taking the drug “as a preventative.”

“Which I think is really kind of crazy, so please don’t do that,” he said at a news briefing. The health department warning said 85% of callers had mild symptoms but that one person “was instructed to seek further evaluation due to the amount of ivermectin reportedly ingested.”

Amid earlier reports of the drug’s use, the National Institutes of Health said most studies on ivermectin’s use against COVID-19 had “significant limitations” and said there was insufficient evidence to recommend either for or against the drug’s use in the pandemic.

An FDA warning said that ivermectin “is not an anti-viral” and that “taking large doses of this drug is dangerous and can cause serious harm.” Although most often used in animals, the FDA said the drug has been approved in smaller doses in humans to treat two conditions caused by parasitic worms.

Mississippi has the nation’s highest per capita outbreak and its lowest rate of vaccination. Although the pace of vaccination has picked up recently in Mississippi, 45.1% of people have received at least one dose compared with the national average of 60.5%, according to the Bloomberg Vaccine Tracker.