TEXAS — For the past week, Dr Gregory Yu, an emergency physician in San Antonio, has received the same daily requests from his patients, some vaccinated for Covid-19 and others unvaccinated: They ask him for ivermectin, a drug typically used to treat parasitic worms that has repeatedly failed in clinical trials to help people infected with the coronavirus.

Dr Yu has refused the ivermectin requests, he said, but he knows some of his colleagues have not.

Prescriptions for ivermectin have seen a sharp rise in recent weeks, jumping to more than 88,000 per week in mid-August from a pre-pandemic baseline average of 3,600 per week, according to researchers from the Centers for Disease Control and Prevention.

Some pharmacists are even reporting shortages of the drug. Mr Travis Walthall, a pharmacist in Kuna, Idaho, a town of about 20,000 people, said that this summer alone he had filled more than 20 ivermectin prescriptions, up from two or three in a typical year. For the past week, he has not been able to obtain the drug from his suppliers — they were all out.

Mr Walthall was astonished, he said, at how many people were willing to take an unapproved drug for Covid-19. “I’m like, gosh, this is horrible,” he said.

Though sometimes given to humans in small doses for head lice, scabies and other parasites, ivermectin is more commonly used in animals. Physicians are raising alarms about a growing number of people getting the drug from livestock supply centres, where it can come in highly concentrated paste or liquid forms.

Calls to poison control centers about ivermectin exposures have risen dramatically, jumping fivefold over their baseline in July, according to CDC researchers, who cited data from the American Association of Poison Control Centers.

Mississippi’s health department said earlier this month that 70 per cent of recent calls to the state poison control center had come from people who ingested ivermectin from livestock supply stores.

Dr Shawn Varney, a toxicologist and medical director for the South Texas Poison Center, said that in 2019 his center received 191 calls about exposure to ivermectin; this year, the center has received 260 calls and is on pace to reach 390 by the end of the year.

The vast majority of the recent calls came from people who took a veterinary product in an attempt to treat or prevent Covid-19.

“Everyone wants some cure for Covid because it’s such a devastating illness,” Dr Varney said. “I plead with people to stop using ivermectin and get the vaccine because it’s the best protection we have at this point. Everything else is risk after risk.”

Dr Varney said people calling the poison control center after taking ivermectin sometimes reported nausea, muscle pain and diarrhea. He noted that there have been ivermectin overdose deaths in the past, although he did not know of any specifically associated with Covid-19.

The biggest risk, he added, comes from people taking the livestock product and ingesting a far higher dose than is appropriate for humans — sometimes 10-15 times the amount that a capsule approved for humans might contain.

“People are going to animal feed stores and getting a formulation that’s highly concentrated because it’s for 1,000-pound animals,” Dr Varney said. “They’re opening themselves to great potential harm.”

Ivermectin was introduced as a veterinary drug in the late 1970s, and the discovery of its effectiveness in combating certain parasitic diseases in humans won the 2015 Nobel Prize for medicine.

Although it has not been shown to be effective in treating Covid-19, people are now clamouring to get the drug, trading tips in Facebook groups and on Reddit. Some physicians have compared the phenomenon to last year’s surge of interest in hydroxychloroquine, although there are more clinical trials evaluating ivermectin.

The Food and Drug Administration weighed in last week. “You are not a horse,” the agency tweeted, with a warning explaining that ivermectin is not FDA-approved for treating or preventing Covid-19 and that taking large doses can cause serious harm.

A recent review of 14 ivermectin studies, with more than 1,600 participants, concluded that none provided evidence of the drug’s ability to prevent Covid-19, improve patient conditions or reduce mortality. Another 31 studies are still underway to test the drug.

One of the largest trials studying ivermectin for Covid-19 treatment, called the Together Trial, was halted by the data safety monitoring board on Aug 6 because the drug had been shown to be no better than a placebo at preventing hospitalisation or prolonged stay in the emergency room.

Dr Edward Mills, a professor at McMaster University who led the study, which enrolled more than 1,300 patients, said the team would have discontinued it earlier were it not for the level of public interest in ivermectin.

“The data safety person said, ‘This is now futile and you’re offering no benefit to patients involved in the trial,’” Dr Mills said.

Researchers and physicians are particularly alarmed by people seeking out ivermectin as a form of possible prevention or treatment instead of getting one of the highly effective Covid-19 vaccines. The FDA last week fully approved the Pfizer-BioNTech Covid-19 vaccine for people 16 and older, and an approval of Moderna’s vaccine is expected in the coming weeks.

“The only functional strategy we have for getting control of Covid-19 is vaccination,” said Dr Irwin Redlener, a physician in New York and founding director of the National Center for Disaster Preparedness at Columbia University.

“If people are not getting vaccinated because of nonsense they’re reading on the internet, that interferes with our ability to get this pandemic under control.”

This article originally appeared in The New York Times.