Play Live Radio
Next Up:
0:00 0:00
Available On Air Stations

How Moldy Hay And Sick Cows Led To A Lifesaving Drug

There is a lifesaving drug that owes its existence to moldy hay, sick cows and rat poison.

The drug is called warfarin sodium. It prevents blood clots, and it can be a lifesaver for patients who've had a heart attack or stroke. It's one of the most widely prescribed drugs in the world.

There is rarely a straight line from idea to invention, but the story of warfarin is particularly twisted. It began on a winter day in the mid-1930s when a farmer showed up at a lab at the University of Wisconsin in Madison.

"He apparently showed up in a blizzard, and he had this can of cow's blood with him," says Kevin Walters, a graduate student in history at the University of Wisconsin who has studied the warfarin story.

The blood came from a cow that had eaten sweet clover hay that had some mold growing in it. The farmer said quite a few of his cows had eaten the moldy hay and had fallen ill with what became known as sweet clover disease.

"The cows eat the hay, and a few days later, they die from bleeding internally, because their blood doesn't clot," says Walters.

The lab the farmer showed up at belonged to a chemist named Karl Paul Link. Link was intrigued by the farmer's problem and decided to try to figure what was happening.

"He redirects his laboratory towards this question of what is it in this hay that is making this cows blood thin out," says Walters.

After years of experiments, in 1940, Link and his colleagues concluded that the blood-thinning molecule arises when a fungus that causes the mold reacts with a natural substance in hay called coumarin. The chemists called the overall blood-thinning compound dicoumarol.

"They realize that the dicoumarol is thinning the cows' blood. Therefore, it could also be used as a blood thinner in human patients," says Walters.

Link and his colleagues collaborated with clinicians who tried dicoumarol as a treatment for patients who had life-threatening blood clots, and it worked. It was regularly used for patients at risk for forming blood clots.

But the chemists weren't done. They started making chemical cousins of dicoumarol. One, labeled No. 42, was a particularly potent blood thinner. But Link didn't immediately turn to its potential human uses; apparently, he was interested in building a better mousetrap — literally. He had tried using dicoumarol to kill rodents in the past, but you can counteract its effects with vitamin K, and rats apparently get a lot of vitamin K in their diets. But No. 42 gave him an idea.

"Maybe if we give [No. 42] to rats ... then what happened to the cows will happen to the rats," says Walters, channeling Link all these years later.

It worked. No. 42 became a popular rat poison, and the active ingredient in the poison was named warfarin. The name was coined by Link. It's a mashup of WARF, the Wisconsin Alumni Research Foundation, and coumarin, the chemical found in sweet clove hay.

So how did a rat poison get back to being a drug? Well, like dicoumarol, warfarin prevents blood clots from forming, and in small amounts, a version called warfarin sodium is much easier to use as a human drug than dicoumarol is. It was used to treat President Dwight Eisenhower after he had a heart attack in 1955.

Warfarin sodium went on to become blockbuster drug under the trade name Coumadin.

"And so that's how you get from a sweet clover disease in cows that is causing cows to bleed internally ... to a blood thinner for humans, then a rat poison, and then a more refined, easier-to-take blood thinner called warfarin," says Walters.

Copyright 2020 NPR. To see more, visit

Joe Palca is a science correspondent for NPR. Since joining NPR in 1992, Palca has covered a range of science topics — everything from biomedical research to astronomy. He is currently focused on the eponymous series, "Joe's Big Idea." Stories in the series explore the minds and motivations of scientists and inventors. Palca is also the founder of NPR Scicommers – A science communication collective.
Madeline Sofia is the host of Short Wave — NPR's daily science podcast. Short Wave will bring a little science into your life, all in about 10 minutes. Sometimes it'll be a good story, a smart conversation, or a fun explainer, but it'll always be interesting and easy to understand. It's a break from the relentless news cycle, but you'll still come away with a better understanding of the world around you.
Meredith Rizzo is a visuals editor and art director on NPR's Science desk. She produces multimedia stories that illuminate science topics through visual reporting, animation, illustration, photography and video. In her time on the Science desk, she's reported from Hong Kong during the early days of the pandemic, photographed the experiences of the first patient to receive an experimental CRISPR treatment for sickle cell disease and covered post-wildfire issues from Australia to California. In 2021, she worked with a team on NPR's Joy Generator, a randomized ideas machine for ways to tap into positive emotions following a year of life in the pandemic. In 2019, she photographed, reported and produced another interactive visual guide exploring how the shape and size of many common grocery store plastics affect their recyclability.