An estimated 30% of people prescribed warfarin will not respond in the expected way due to genetic variation. Current data is clear that variants of the CYP2C9 and VKORC1 genes influence individual warfarin dose requirements and bleeding event risk. Such variants explain an estimated 40% of the inter-individual variability in warfarin response.

FDA Revises Warfarin Labeling

In August 2007, the FDA revised the warfarin package insert to highlight the usefulness of CYP2C9 and VKORC1 gene variant testing to predict individual warfarin dose requirements. They advise that lower initial doses should be considered for people with certain gene variants.