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.

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