Tecarfarin is a vitamin K antagonist under development for use as an anticoagulant. A Phase II/III clinical trial in 607 people, comparing it to the established vitamin K antagonist warfarin, found no difference in quality of anticoagulation or side effects between the two drugs in the overall population. Among patients taking CYP2C9 interacting drugs however, the tecarfarin patients’ TTR was 72.2% (n=92) vs 69.9% (n=87) for warfarin patients (pint=0.16); among patients who had both a CYP2C9 variant allele and taking a CYP2C9 interacting drug, TTR was 76.5% and 69.5% for the tecarfarin (n=24) and warfarin (n=31) groups, respectively (pint=0.24). This study included in 84 (14%) patients with a mechanical heart valve as an indication for anticoagulation therapy. No thrombotic or embolic even