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FDA Warns Against Taking Heartworm Drug to Prevent, Treat COVID-19

Misuse of ivermectin intended for horses has led to hospitalizations

Ivermectin, 3 mg tablet, sitting on a prescription label.

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En español | The Food and Drug Administration (FDA) is warning people against taking ivermectin, a medication used to treat heartworm in animals, as protection against developing COVID-19, saying it could endanger them.

"The FDA has received multiple reports of patients who have required medical support and been hospitalized after self-medicating with ivermectin intended for horses,” the federal regulatory agency said in a press release.


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The FDA has approved the use of ivermectin tablets at specific dosages to treat people with intestinal strongyloidiasis and onchocerciasis, two conditions caused by parasitic worms. The drug is also used to treat heartworm in animals, but those dosages – particularly for large animals like horses – can cause severe harm if taken by people.

"Animal drugs are often highly concentrated because they are used for large animals like horses and cows, which can weigh a lot more than we do — a ton or more. Such high doses can be highly toxic in humans,” the FDA says.

The agency notes that ivermectin should not be taken without medical supervision and only for approved purposes, even at low concentrations. It also stresses that people should not take medications intended for animals.

"Even the levels of ivermectin for approved uses can interact with other medications, like blood-thinners. You can also overdose on ivermectin, which can cause nausea, vomiting, diarrhea, hypotension (low blood pressure), allergic reactions (itching and hives), dizziness, ataxia (problems with balance), seizures, coma and even death,” the FDA says.

Animal drug untested in humans as COVID-19 treatment

Early studies showed that ivermectin can inhibit the replication of the novel coronavirus, SARS-CoV-2, in cell cultures, but to achieve the same level of antiviral effectiveness in a human would require administering doses up to 100 times higher than those approved by the FDA, according to the National Institutes of Health (NIH).

So far, the NIH says there is not enough information for its panel of COVID-19 treatment experts to recommend for or against ivermectin's use as a prophylaxis.


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Merck, the pharmaceutical manufacturer of Stromectol, the brand name of ivermectin, also says the medication should be used only for FDA-approved purposes. Specifically, the company said in a press statement issued in February, it had, to date, found:

  • No scientific basis for a potential therapeutic effect against COVID-19 from pre-clinical studies
  • No meaningful evidence for clinical activity or clinical efficacy in patients with COVID-19 disease
  • A concerning lack of safety data in the majority of studies

Ivermectin's use as a potential prophylactic has been touted by a group called the Frontline COVID-19 Critical Care Alliance, which claimed studies indicated the medication demonstrated effectiveness in preventing COVID-19 infection prior to and after exposure to the coronavirus. The group, made up primarily of critical care doctors, held a press briefing on Dec. 4 in Houston calling on the NIH to recommend the drug for use against COVID-19.

Peter Urban is a contributing writer and editor who focuses on health news. Urban spent two decades working as a correspondent in Washington, D.C., for daily newspapers in Connecticut, Massachusetts, Ohio, California and Arkansas, including a stint as Washington bureau chief for the Las Vegas Review-Journal. His freelance work has appeared in Scientific American, Bloomberg Government and CTNewsJunkie.com.

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