Warfarin, molecular model. Warfarin (C19H16O4) is an anticoagulant drug used to treat clotting disorders. Atoms are represented as balls and are colour coded: carbon (grey), oxygen (red) and hydrogen (silver). Warfarin was originally produced as a rat poison in 1948. Its name derives from the Wisconsin Alumni Research Foundation, where it was produced. It was not until 1954 that its use as a therapeutic anticoagulant in humans was recognised. It is now little used as a rat poison, as many rat populations have developed resistance to it.