The authors report a clinical case of warfarin hypersensitivity noticed when a patient, with thromboembolic risk, was submitted to warfarin. She was found to be heterozigotic to -1639G>A (gene VKORC1) and homozygotic to 1075A>C (genotype CYP2C9*3/*3). In this article the warfarin pharmacodinamics and its role over Vitamin K is revisited. It includes some suggested indications for investigating genetic polymorphisms, in order to avoid complications associated with warfarin.