Clinical trials have repeatedly failed to show that the drug is effective against the virus. Still, U.S. demand is high.,

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Public health warnings against using the anti-parasitic ivermectin as a treatment for Covid, especially in the large doses meant for livestock, appear to have made little progress in stemming its popularity in parts of the United States.

Hospitals and poison control centers across the United States are treating a growing number of patients taking the drug.

The Centers for Disease Control and Prevention reported that almost 90,000 prescriptions for ivermectin were being written per week in mid-August, up from a prepandemic weekly average of 3,600. Veterinary supply store shelves have been emptied of it.

The C.D.C. reported that one person had an “altered mental status” after purchasing ivermectin on the internet and taking a reported five pills daily for five days. Another person drank an ivermectin formula intended for cattle and was hospitalized for nine days with tremors and hallucinations.

The warnings are not enough. Ivermectin has been promoted by celebrities like the podcasting giant Joe Rogan, who listed it this week among the treatments he was given after contracting the virus. Alex Jones, the conspiracy theorist who has been banned from Facebook, YouTube and Twitter, took out a box of ivermectin pills during one of his trademark rants and popped two tablets live on the show that he still manages to stream.

Ivermectin was introduced as a veterinary drug in the late 1970s, and it was later approved for use in humans infected with parasites. Since 1987, its U.S. maker, Merck & Co., has donated billions of doses that have spared hundreds of millions of people from river blindness in Africa and other parasitic diseases. Two scientists shared a Nobel Prize in 2015 for their work in developing the drug. But in the United States, it has limited human applications, like treating lice and rosacea, and has been primarily used to deworm horses, cows and pets — until now.

On Twitter last month, the Food and Drug Administration warned that the drug was not approved for use against Covid and that taking large doses could cause serious harm. “You are not a horse,” its tweet read. “You are not a cow. Seriously, y’all. Stop it.”

In a statement on Wednesday, alarmed health experts from the American Medical Association, the American Pharmacists Association and the American Society of Health-System Pharmacists called for “an immediate end to the prescribing, dispensing and use of ivermectin for the prevention and treatment of Covid-19 outside of a clinical trial.”

Though it has not been shown to be effective in treating Covid, people are still clamoring to get the drug, trading tips in Facebook groups and on Reddit. Some physicians have compared the phenomenon with last year’s surge of interest in hydroxychloroquine, though ivermectin has undergone more clinical trials.

One of the largest trials, called the Together Trial, was halted last month because the drug had been shown to be no better than a placebo at preventing hospitalization for Covid. In July, a research paper indicating that the medicine reduced Covid deaths was withdrawn after questions arose about plagiarism and data manipulation.