Warfarin anticoagulant drug, molecular model. Warfarin is an anticoagulant used to treat blood clotting disorders. Warfarin's name derives from the Wisconsin Alumni Research Foundation, where it was produced. It was introduced as a rat poison in 1948 and it was not until 1954 that it was recognised as a potential therapeutic anticoagulant in humans. It is now rarely used as rat poison, since many rat populations have become resistant to it. Warfarin's chemical formula is C19H16O4. Atoms are represented as spheres and are colour-coded: carbon (C, grey), hydrogen (H, turquoise) and oxygen (O, red).