 Warfarin was patented in 1948 and was introduced commercially in 1954. In 2004, more than 24 million prescriptions were written for warfarin in the United States alone. Warfarin constitutes the 20th most frequently prescribed drug in the United States. This is remarkable, because few drugs have had a life cycle as long as warfarin, and even fewer can claim an increase in use more than 50 years following introduction. 