10-hydroxy Warfarin is a metabolite of (R)-warfarin .1It is formed from (R)-warfarin by the cytochrome P450 (CYP) isoform CYP3A4. 10-hydroxy Warfarin is an inhibitor of CYP2C9 (IC50= 1.6 μM), the enzyme that converts (S)-warfarin to its 6-hydroxy warfarin and 7-hydroxy warfarin metabolites.2