Warfarin was the anticoagulant first sold as rat poison before receiving U.S. medical approval in 1954.
Its story began with research into bleeding cattle that had eaten moldy sweet clover hay. Scientists traced the hemorrhaging to coumarin-related compounds, leading Karl Paul Link and his University of Wisconsin team to develop more potent anticoagulants. Warfarin was patented in 1945 and registered as a rodenticide in the United States in 1948.
The compound works by interfering with vitamin K recycling, reducing the body’s ability to produce several clotting factors. That same mechanism makes it useful against dangerous clots but requires careful dosing and monitoring, commonly with the INR blood test.
A common mix-up is confusing warfarin with heparin. Heparin is another anticoagulant, but it was not the rat poison in this story and is chemically and clinically distinct. Warfarin’s name comes from WARF, the Wisconsin Alumni Research Foundation, plus the suffix associated with coumarin.