Thrombosis, which has been defined as blood clotting in the wrong place, is among the principal causes of death in the Western world. One of the drugs that is most frequently used to prevent arterial thrombosis is the oral anti-coagulant, warfarin, which was first marketed as rat poison in the 1940s. In 1999, warfarin was the eleventh most prescribed drug in the USA, with annual sales of approximately US$500 m.

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