Why Are People **REALLY** Taking Horse Dewormer for COVID-19?

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Last month, one of my patrons (hi Chris!) asked during my Patreon Q&A livestream “what’s the deal with Ivermectin?” And I was really annoyed because I had no idea at the time and it sent me on a deep dive into why a horse dewormer was the talk of the town, and the monthly Q&A is not supposed to require that much work, dammit. Ask me my favorite Triscuit flavor! It’s black pepper. See? Wasn’t that fun?

Patrons asked if I would turn my answer in the Q&A into an official video, so, here we are. Hello.

So why did an anti-parasite medication for horses become a commonly used treatment against a virus in humans? A virus isn’t a worm, right? A horse isn’t a human, right??

Well. For a start, just because a medication treats something in one species doesn’t mean it can’t treat something completely different in another species, so while it’s objectively hilarious to mock people who say they won’t take the vaccine because they don’t know what’s in it and instead prefer to eat horse dewormer paste, it’s not actually a rational counter argument. Pfizer researchers studied an enzyme-blocking drug called sildenafil as a treatment for high blood pressure and chest pain, but now a bunch of people take it to get boners and you know it as Viagra. Thalidomide was used in the 1950s as a sleep aid and anti-nausea drug, until it was found to cause severe birth defects when taken by pregnant women. In 1964 a doctor happened to give thalidomide to a patient with leprosy and found that it helped the painful skin condition. And later, doctors discovered that it could also be a treatment for some cancers like myeloma. How does one drug cure nausea, cause birth defects, heal leprosy sores, and treat cancer? Researchers don’t really know. That’s right, the exact “mechanism of action” of thalidomide is still unclear despite thousands of studies attempting to figure it out.

So, just because a medication isn’t known to help with a condition doesn’t mean it can’t. And it’s possible for a medication to help with a condition even if researchers don’t know how it works.

Ivermectin has been around since the ‘70s as an anti-parasite drug for both humans AND other animals. The reason why you see it referred to as a horse (or sheep or cow) dewormer is simply because pharmacies have been running low on it, so people have been getting it from feed supply stores. It’s the same stuff but generally humans need way less than horses.

It works by attacking nerve cells found in the bodies of invertebrates, which are luckily NOT found in the bodies of vertebrates like you or a horse or a sheep.

However, over the years scientists have tested ivermectin for its potential antimicrobial and antiviral properties. For instance, in 2012 researchers found that in a petri dish, ivermectin interfered with the replication of the yellow fever virus. Further testing revealed that it also slowed the replication of dengue fever, Japanese encephalitis and tick-borne encephalitis viruses. All of those viruses have something in common: they are from the genus Flavivirus, all single-strand RNA viruses. Zika is also in that genus, which is why ivermectin was also tested against that in 2016 and found (again, in a petri dish) to be effective. The precise mechanism of action was unknown.

Over the years there have been a handful of other viruses that ivermectin seemed to prevent from replicating in petri dishes, nearly all of them single-stranded RNA but a few that are DNA viruses, like one specific strain of horse herpes. I’m sorry, I know I made a whole video about how we need to stop shaming people for having herpes but the phrase “horse herpes” is very funny.

Anyway, all of those studies with positive results were done “in vitro,” meaning in a test tube or a petri dish and not in a living organism. While there have been a few “in vivo” studies (mice, basically), the results were nowhere near as good and they still required massive amounts of ivermectin.

And all that is why back in June of 2020, some doctors threw together a systematic review of the studies looking at ivermectin’s effects on viruses. Their conclusion was this:

“As noted, the activity of ivermectin in cell culture has not reproduced in mouse infection models against many of the viruses and has not been clinically proven either, in spite of ivermectin being available globally. This is likely related to the pharmacokinetics and therapeutic safety window for ivermectin. The blood levels of ivermectin at safe therapeutic doses are in the 20–80?ng/ml range [44], while the activity against SARS-CoV2 in cell culture is in the microgram range. Ivermectin is administered orally or topically. If safe formulations or analogs can be derived that can be administered to achieve therapeutic concentrations, ivermectin could be useful as a broad-spectrum antiviral agent.”

So essentially, “here’s some evidence that ivermectin might help against this pandemic but we need way more research and we probably need to change it so that it will work without doing more harm than good.”

I would now like to illustrate what happened next by use of this helpful meme.

Researchers quickly were able to show that ivermectin did reduce replication of SARS-CoV-2 in a petri dish, which meant the important next step was to show it worked in a living organism.

Over the next year, dozens of studies were launched to do just that. About a dozen were published, and only a few of those were of decent size and quality. In May of 2021 a Cochrane review was published saying that absolutely nothing was learned but that “Overall, the reliable evidence available does not support the use of ivermectin for treatment or prevention of COVID?19 outside of well?designed randomized trials.”

Except! Early on, there was one large study that showed a positive result and it was extremely exciting. Researchers at Egypt’s Benha University followed 400 people with COVID-19 symptoms and found that ivermectin reduced the death rate by an astonishing NINETY PERCENT. A preprint of the paper (meaning it hadn’t undergone peer review yet, which has been exceedingly common during the pandemic) went online in November of 2020. Despite the lack of peer review or, you know, getting published in a real journal, that study was cited by more than 30 other papers and officially kicked off an ivermectin craze.

Unfortunately, it appears that the paper’s data was tampered with, plagiarized, and/or just completely made up. According to, the data didn’t match what the authors claimed, like for instance where the authors claimed that four out of 100 people died in the standard COVID treatment group but the data says that number is 0, and they said that two out of 100 died in their ivermectin group but the data says that number is 4. That…that seems important?

Multiple patients in the raw data were clearly copied and pasted since they even included typos like “coguh” instead of “cough.” There are a million other red flags detailed by but the crux of it is this: thousands of people are taking ivermectin instead of getting vaccinated because of an obviously fraudulent “study” that was retracted before it was even published anywhere. Poison control calls are up 550% in Texas and there are Facebook pages full of people who are astounded that worms are now appearing in their poop, when in fact the “worms” are more likely to be stringy bits of their intestinal lining that is sloughing off because they’re taking concentrated doses of horse dewormer. I wish I could go back in time and tell the Rebecca of 15 years ago that not only did she fail to make the world a more rational place but that in 15 years she would say that previous sentence, which I shall not repeat.

Anyway, that’s how we got to where we are! There ARE still clinical trials of ivermectin treatment for COVID-19 ongoing, but I wouldn’t hold your breath. I WOULD get a vaccine, but what do I know? I like my intestinal lining where it is, thank you very much.

Rebecca Watson

Rebecca is a writer, speaker, YouTube personality, and unrepentant science nerd. In addition to founding and continuing to run Skepchick, she hosts Quiz-o-Tron, a monthly science-themed quiz show and podcast that pits comedians against nerds. There is an asteroid named in her honor. Twitter @rebeccawatson Mastodon Instagram @actuallyrebeccawatson TikTok @actuallyrebeccawatson YouTube @rebeccawatson BlueSky

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  1. Wow, the cognitive dissonance for people who won’t get a vaccine because it isn’t approved by the FDA, but will take horse paste, even though it isn’t approved by the FDA, must be brain-addling. Of course, this assumes people eating horse paste have brains, which…

    1. Their brains were eaten by worms… Ivermectin works for horses and sheep’s intestinal parasites, but not for brain worms in primates.

  2. Yeah it was the same with HCQ. it seemed to work fine so long as you were a green monkey cell culture and had Sars 1 but for the current problem in humans, not so much.
    Heaps of things seem to work in cell cultures but between that and an effective treatment there is at least a year of Phase 1, 2 and 3 trials.
    Which the vaccines have ALREADY completed, dumbasses!

  3. One thing to add to this story is that another flawed ivermectin preprint had a major influence early in 2020. Since outside the US, the ivermectin craze took off well before November 2020 (when the Benha university paper went online).

    As early as April and May 2020, ivermectin use was surging in Peru, Bolivia, Guatemala, Paraguay and other Latin American countries. For all the reasons you outlined, but also because of an April 2020 preprint on SSRN that suggested ivermectin could reduce coronavirus-related deaths in people. By May, Peru’s ministry of health had recommended the use of ivermectin, following the production of a policy paper that cited the SSRN preprint. Bolivia’s government and regional authorities in Natal, Brazil, recommended it too.

    That SSRN preprint, however, had been published by researchers who thought they were using data from Surgisphere, the US firm that had supposedly collected health records from hospitals around the world. These Surgisphere-collated records were also used for two high-profile research studies in NEJM and The Lancet. But by June, it turned out no-one had seen the data for any of these studies and Surgisphere wouldn’t release the data. This led to the retraction of the NEJM and Lancet papers, and also, more quietly, the removal of the SSRN preprint.

    From and

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