The war against COVID 19 includes the search for safe and effective medicines to treat those who contract the virus. A recent breakthrough from Merck provides hope for many.
Meanwhile off-label use of an older Merck drug referred to as a horse dewormer has been a hot news item. The route a chemical compound takes from isolation, to identification, to discovery of therapeutic properties, and finally validation of safety and efficacy can be a long journey with many twists and turns along the way. The paths taken by two chemical compounds to become potential COVID treatments are both stories that touch upon horses. While horse owners know of ivermectin and use it regularly, they may be unaware that ivermectin is not only a veterinary drug, but also has been formulated and used to treat certain human conditions. Not only does ivermectin have a history connected to equine medicine, but Merck’s newest potential medicine to combat coronavirus has a link as well.


The story of the development of ivermectin begins with collection of soil samples in the 1970’s in Japan. Bacteria were isolated from those soil samples soil samples by Merck scientists, and then a class of active compounds (avermectins) were discovered as products of the bacteria. After recognizing the avermectins’ abilities to vanquish parasites, a formulation known as ivermectin was created and then sold widely beginning in the 1980’s for veterinary use. It has a few different forms for use in livestock, but horse owners are most familiar with the oral paste version used to regularly deworm their equines.

After gaining wide acceptance as a veterinary medicine, scientists learned that ivermectin was just as capable of treating certain parasites in humans. The first clinical trials with patients ran in the early 1980’s, and a Nobel Prize for the research on the avermectins was awarded in 2015. So although many refer to ivermectin as a “horse dewormer drug”, and its effectiveness is not yet validated against the COVID virus, the active agent does have FDA approval for human use in tablet form to treat River Blindness, among other scourges. Clinical trials are currently underway to gauge the effectiveness of the drug for defeating the current pandemic. So the drug is not yet approved for a COVID indication, but doctors can and do prescribe it for off-label use. One example is Joe Rogan, a well-known podcaster who has been prescribed the pills. Unfortunate misreporting led some people to believe he had taken the veterinary paste, while in fact he took the human medicine version. To avert the controversy and confusion swirling around the human tablet formulation versus the veterinary paste formulation, the FDA and CDC have issued strongly worded guidance that the ivermectin veterinary paste should not be ingested by humans.

As a side note, since it is known that the coronavirus family of viruses can also afflict animals, it would be interesting to study whether dewormer paste given to horses treats COVID at the same time. This could be an exciting new chapter in the ivermectin story.


Molnupiravir is the other Merck drug advancing as a COVID treatment. Although the name seems like a mouthful, it is actually infused with great meaning. Thor’s mythical hammer Mjolnir inspired the compound’s name, embodying the hope that the molnupiravir could strike a strong blow against the virus. This second Merck drug and potential COVID-19 treatment also has a back story of its discovery that involves horses. Merck has just applied for FDA Emergency Use Authorization to sell molnupiravir as an oral treatment against COVID-19 in the US, based on positive results obtained from ongoing Phase 3 clinical trials. The equine connection to this potential breakthrough in the fight against the virus lies in its early development. The drug was originally identified during an Emory University project funded by the U.S. Defense Threat Reduction Agency as an antidote for Venezuelan equine encephalitis virus (VEE). The mosquito-borne virus VEE had been proposed as a potential biological warfare agent, and an antidote for it was desired, in the event it were to be actually deployed. During testing, it was noticed that the compound could be also used against coronaviruses. The discovery was licensed to Merck, to bring it forward as a COVID treatment.

In the words of radio host Paul Harvey – now you know the rest of the story. For other interesting accounts of products for horses that went on to be marketed for human use, read my previous article “Products that Jumped from the Tack Shop to Pharmacy Shelves” here.

Elaine Marie

I’m a licensed patent attorney and partner at Flener IP & Business Law LLC in Chicago, Illinois.

As a horse owner and avid equestrian, I’m a passionate advocate for animal owners; particularly horse owners. I’m dedicated to the advancement of the equestrian way

I’m a licensed patent attorney and partner at Flener IP & Business Law LLC in Chicago, Illinois.

As a horse owner and avid equestrian, I’m a passionate advocate for animal owners; particularly horse owners. I’m dedicated to the advancement of the equestrian way of life and have participated in regional, county and local comprehensive planning processes as an adviser to ensure that land use options that allow horse keeping are not overlooked.  As an elected representative to my Village, I gained local government experience.

I have the ability to dissect complex issues aided by a strong scientific and legal background. On a pro bono basis, I have drafted four bills which became Illinois state laws; and successfully advocated for passage of bills I authored as well as several other new Illinois pro-equine owner laws.  And I led a local award-winning public-private partnership effort to raise funds for a horse trailer parking lot to support additional public bridle trails in her area, including preparation of successful grant applications to help fund the $200,000 project. 

My horse-related articles can be found in the Midwest Equestrian magazine.