Warfarin molecule. Computer graphic of the anti- coagulant drug warfarin (formula C18.H16.O3). The atoms (spheres) are colour-coded: carbon (yellow), oxygen (red) & hydrogen (white). Warfarin was invented by Karl Paul Link in 1939. It is prescribed to prevent thrombosis in high risk patients. A derivative is used as a rat poison that works by stopping their blood from clotting, so that they bleed to death from minor wounds.