Interest in ivermectin, a common antiparasitic drug, as a treatment for Covid-19 has surged in recent months, but several large studies indicate that its effect on the disease are either small or nonexistent.

Cheat sheets: Evidence-based medicine 101

What is ivermectin?

Ivermectin is an anti-parasite drug, which was first discovered in 1975, Vox reports. It was initially introduced as a drug for livestock in the 1980s, and a version of the drug for humans was developed later.

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According to NPR, ivermectin is used to treat several parasitic infections around the world, including river blindness and elephantiasis. Around 250 million people take ivermectin to treat parasitic infections every year, and its side effects, such as nausea, itching, and rashes, are typically mild.

Although ivermectin is not an anti-viral drug, many researchers at the start of the pandemic began testing existing drugs to see if they could be repurposed as treatments for Covid-19. According to Vox, since most drugs can work through many different mechanisms in the body, it is not uncommon for existing drugs to work against diseases for which they were not originally developed.

Early on, several small studies on ivermectin suggested the drug could be effective as a treatment against Covid-19. For example, a study published in Science Direct last year found that high doses of ivermectin were able to effectively neutralize the coronavirus in laboratory cell cultures. However, since then, several larger, well-conducted studies have suggested that any benefits of the drug are small or that it has no effect on patients, Vox reports.

Ivermectin interest rises amid the delta surge

In December 2020, Pierre Kory, a critical care physician and founding member of the group Front Line Covid-19 Critical Care Alliance, testified at a Senate committee hearing about the benefits of ivermectin.

According to Kory, ivermectin was "proving to be of miraculous impact." He also claimed that the drug could completely prevent infection from the coronavirus. "It basically obliterates transmission of this virus," he said—a claim that was not backed by scientific evidence.

Kory's claims spread widely online, NPR reports, and as the delta variant began to cause cases to surge across the United States, interest in ivermectin began to grow again.

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On June 22, popular podcast host Joe Rogan invited Kory and Bret Weinstein, an evolutionary biologist, to discuss ivermectin on his podcast. On the podcast, Weinstein said ivermectin was "good enough to end the pandemic at any point you wanted," an assertion that is not supported by research.

After the episode of the podcast aired, interest in ivermectin as an alternative Covid-19 treatment surged, Vox reports. Based on CDC data, 88,000 prescriptions for ivermectin were written in a single week in mid-August, compared to 15,000 the week before Rogan's episode released and 3,600 a week before the pandemic began.

In addition, NPR reports that online mentions of ivermectin have jumped significantly, with 74% of the drug's mention taking place in the last two months alone.

Demand for the drug has also increased, the New York Times reports—so much so that some pharmacists have reported shortages of the drug.

And some people who cannot access the drug—either because their providers will not prescribe it for them or another reason—have turned to the veterinary version of the drug.

Unlike ivermectin that has been formulated for humans, veterinary ivermectin is highly concentrated and can contain a dose that is 10 to 15 times higher than the amount approved for human use, the Times reports. In addition, usage of veterinary versions of ivermectin have led to a dramatic rise in calls to poison control centers, prompting FDA to urge people to stop using the drug to treat or prevent the coronavirus.

What research says about the drug's effectiveness against Covid-19

According to the National Institutes of Health's (NIH) Covid-19 treatment guidelines, there is currently not enough evidence "to recommend either for or against" use of the drug for Covid-19 patients.

In particular, a study published in JAMA found that ivermectin did not reduce the duration of symptoms in patients with mild cases of Covid-19. In the study, 400 patients with mild Covid-19 symptoms were either assigned to a five-day course of ivermectin or a placebo. The researchers found that on average, patients who received ivermectin had symptoms lasting 10 days compared to 12 days for patients who received the placebo—a difference that was not statistically significant.

"It appears to be a safe medication, but that is not enough to prescribe it openly," Eduardo López-Medina, a researcher at the Center for Pediatric Infectious Diseases in Colombia who led the study, said. "People should use it in trials but not necessarily to treat patients. The data is not robust enough to support its use."

A separate study of 1,300 patients, which was presented at an NIH roundtable and has not yet been published, was halted on Aug. 6 by a data safety monitoring board after the drug was shown to be no better at preventing hospitalization or reducing ED stay duration than a placebo, the Times reports. According to Edward Mills, a professor at McMaster University who led the trial, the study found "no important clinical benefit" of ivermectin.

Although some large-scale studies have suggested ivermectin has significant benefits for Covid-19 patients, Gideon Meyerowitz-Katz, an epidemiologist at the University of Wollongong in Australia, said such studies are "probably fraudulent."

For example, at least two influential preprint studies that suggested ivermectin's benefits were later retracted after it was found that they contained flawed or manufactured data, NPR reports.

Instead, Meyerowitz Katz said most high-quality studies show that "confidence intervals span both modest benefit and modest harm."

Separately, a recent meta-analysis of 14 ivermectin studies published in the Cochrane Database of Systematic Reviews, which only included high-quality studies with sound methods and data, found that there is not enough rigorous evidence to suggest that ivermectin prevents Covid-19, improves patients' conditions, or reduces the risk of death.

"There is great interest in repurposing well-known inexpensive drugs such as ivermectin that are readily available as an oral tablet," Maria Popp and Stephanie Weibel, two of the meta-analysis' authors, said. "Even if these circumstances seem ideal, the results from the available clinical studies carried out so far cannot confirm the widely advertised benefits."

Separately, Shawn Varney, a toxicologist and medical director at the South Texas Poison Center, said, "Everyone wants some cure for Covid because it's such a devastating illness. 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." (Piper, Vox, 9/17; Goldberg, New York Times, 9/13; Huang, NPR, 9/19; Anthes, New York Times, 9/7)