In the 1920s, a mysterious disease was killing cattle in the United States and Canada. The cattle were bleeding to death from minor injuries or even spontaneously. 

Researchers discovered that the cattle had been eating moldy sweet clover hay, which contained a substance that prevented their blood from clotting properly. In 1940, a chemist named Karl Paul Link at the University of Wisconsin isolated the compound responsible for this effect, which was eventually named dicoumarol. 

Further research led to the development of warfarin (named after the Wisconsin Alumni Research Foundation, WARF), which was initially used as a rat poison. However, in 1951, after a Navy recruit attempted suicide by ingesting warfarin and survived after receiving vitamin K, researchers realized that warfarin could be used safely as a blood thinner in humans when properly dosed. 

This discovery marked the beginning of warfarin's use as a medication to prevent blood clots in patients.