Coumadin is the most widely prescribed blood thinner in the world today, used to treat heart patients and prevent blood clotting. The drug was derived from the anticoagulant Warfarin (named after WARF), a popular rodent poison developed in 1941 by Professor Karl Paul Link of the UW-Madison School of Agriculture. Link and his team isolated the compound from sweet clover hay after a farmer approached them with samples of non-coagulated blood from dying cattle.