The requests were strange: Customers flocking to a Nevada feed store were asking for an animal deworming drug that they said worked for covid-19. “‘No, that’s not for you,’” Makenna LaFond, who works at Sierra Feed and Saddlery in Reno, recalled saying. “‘That’s for a 1,100-pound horse.’ Then, they would buy, like, six tubes of it.”

LaFond said requests for the paste decreased late last year after she aired her concerns during a local television interview. But elsewhere, interest in ivermectin is running high, despite insufficient evidence that it works as a treatment for covid-19 and the sometimes dangerous consequences when people take the animal version. The Food and Drug Administration said at least three people were hospitalized in February after taking the veterinary formulation. It warned that high doses can cause allergic reactions, seizures, liver injury and even death.

Experts say some people are snapping up the animal drug because it is easier to obtain than the one designed for people. That formula is approved for tropical maladies and requires a prescription. And although ivermectin is touted on social media as something that could end the pandemic, it has not been cleared for use against covid-19, the disease caused by the novel coronavirus.

“It’s like the new hydroxychloroquine,” said Angela Rasmussen, a virologist at Georgetown University’s Center for Global Health Science and Security, referring to the malaria drug pushed by President Donald Trump that proved ineffective against covid-19. “It would be great if ivermectin did work — it’s been around for years and is cheap. But to my knowledge, there is no data that suggests it’s good for covid-19.”

Ivermectin is highly effective in treating devastating parasitic infections such as river blindness and is safe when used at recommended doses. In the 1970s, Japanese and U.S. scientists discovered the compound that led to its development, and in 2015 received the Nobel Prize. But against covid-19, there is “no meaningful evidence” of efficacy, according to Merck, which makes the brand-name version. And the nation’s leading organization of infectious-disease doctors recommends against using ivermectin outside of clinical trials.

But Pierre Kory, a critical-care physician, is undeterred. Kory, a co-founder of the Front Line Covid-19 Critical Care Alliance, a group of physicians and scientists from several countries, argues that studies from around the world show ivermectin is a “miracle drug” for covid-19 and should be deployed immediately. He opposes waiting for data from large randomized clinical trials to authorize its use, saying too many people are dying of covid-19.

Now, the National Institutes of Health might wade into the controversy. It is planning a randomized clinical trial to explore whether older, already approved drugs can be repurposed to reduce covid-19 symptoms, according to three individuals who spoke on the condition of anonymity because the plans have not been announced. Ivermectin is considered a top candidate for the trial, though the details are not final, the individuals said. Other possibilities are fluvoxamine, a decades-old antidepressant, and famotidine, the generic name for Pepcid, outside scientists said. The goal would be to get results within months.

The outpatient trial would be the latest in a series of studies, conducted with academia and industry, in NIH’s Accelerating Covid-19 Therapeutic Interventions and Vaccines program. The initiative has scrutinized monoclonal antibodies, blood thinners and other agents. Such trials, experts say, are as important in determining what doesn’t work, as what does.

A British trial called Recovery has provided important information about older drugs, finding that hydroxychloroquine did not help hospitalized covid-19 patients, while the steroid dexamethasone could be a lifesaver. But the United States has lagged behind in conducting such trials because of the fragmentation of the health-care system and the lack of financial incentive for pharmaceutical companies to continue to do research on old, cheap drugs.

Interest in ivermectin for covid-19 surged last spring after a small Australian study found that the drug inhibited the replication of the coronavirus in laboratory tests. But many scientists warned that it would be impossible for humans to take high enough doses of the drug to produce an antiviral effect.

Sales of the veterinary version accelerated, making it harder for animal owners to find ivermectin for pets and livestock. Tractor Supply, a national retailer of farm products, temporarily stopped selling one form of the animal drug to prevent improper use. It has since resumed selling the product — “carefully labeled,” the company said in a statement.

“We don’t have solid evidence right now” on the drug, said Janet Woodcock, acting FDA commissioner, during a recent webinar on covid-19 treatments sponsored by the American Medical Association. “We’ve had hints for a long time on ivermectin, but again, we’ve had hints on a lot of these agents and many of them have not panned out.”

Patti Gilliano, a 56-year-old nurse who lives near Houston, thinks that ivermectin was instrumental in her recovery from covid-19 last summer. After she was hospitalized, she said, she was treated with the antiviral drug remdesivir. But it wasn’t until she got home and found a doctor to prescribe ivermectin, she said, that the brain fog, night sweats and gastrointestinal issues disappeared.

Scientists warn, however, that such anecdotes do not prove a treatment works, because most people who get covid-19 eventually recover.

Kory, of Critical Care Alliance, said the doctors group was formed at the start of the pandemic to trade information about potential treatments and was an early advocate of using steroids, now part of the standard of care for seriously ill patients. Last fall, it began focusing on ivermectin as several studies emerged from countries in Southeast Asia and South America suggesting that it could be beneficial in treating covid-19.

In emotional testimony before the Senate Homeland Security Committee in December, Kory extolled the drug as a “potential global solution” to the pandemic. After his testimony attracted widespread attention, Kory said, his employer — Aurora St. Luke’s Medical Center in Milwaukee — tried to impose new contract restrictions that he claims would have interfered with his free speech. He refused to accept the restrictions and left. The hospital declined to comment.

Previously, he worked in the health system for the University of Wisconsin at Madison but left that job last May, he said, because his superiors refused to follow his recommendation that covid-19 patients be treated with steroids. That was a month before the first big clinical trial — the British Recovery trial — showed the value of the steroid dexamethasone. The health system declined to comment.

Kory said he now works as a “locum tenens” physician — a doctor who works in temporary jobs — and as president and chief medical officer of the Front Line Covid-19 Critical Care Alliance.

Last August, the NIH committee that issues covid-19 treatment guidelines recommended against the use of ivermectin outside of clinical trials. But after a presentation by Kory and his colleagues in January, the panel adopted a neutral stance, saying there was “insufficient data to recommend either for or against the use of ivermectin for the treatment of covid-19.” It said some studies showed benefits and others did not. But it noted that most of the studies “had incomplete information and significant methodological limitations, which make it difficult to exclude common causes of bias.”

Several weeks later, the lnfectious Diseases Society of America, made up of infectious-disease physicians, said most of the ivermectin studies were too small, weren’t properly randomized or had other flaws, and said the drug should not be used outside of trials. Adarsh Bhimraj, an infectious-disease doctor at the Cleveland Clinic and a member of the treatment-guidelines panel, pointed to what he described as a well-designed, randomized, placebo-controlled trial in Colombia that found the drug did not help patients with mild covid-19. The study was published in the Journal of the American Medical Association in early March.

The rush to embrace unapproved therapies reflects an understandable desire to find a silver bullet for the viral menace, Bhimraj said. While some progress has been made on covid-19 treatments, there haven’t been any game-changers like penicillin’s ability to kill streptococcus bacteria, he noted.

Amid the debate, some doctors are trying to turn down the temperature.

In a blog post earlier this year, Paul Sax, an infectious-disease physician at Brigham and Women’s Hospital in Boston, acknowledged the debate over ivermectin, with its echoes of the battle over hydroxychloroquine, provokes “a weary feeling of deja vu” in many doctors. But he warned physicians against assuming that “because we were burned by hydroxychloroquine means that all other repurposed anti-parasitic drugs will fail too.”

He expressed cautious optimism that the drug might have a benefit, based largely on a recent meta-analysis — a statistical analysis based on several studies — by the University of Liverpool. The data, Sax said, “look stronger than they ever did for hydroxychloroquine, but we’re not quite yet at the ‘practice changing’ level.”

Alice Crites contributed to this report.

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