This study is a crossover randomized controlled equivalence trial. It comprises four phases, each one one-month long, being the first one a run-in period. Patients will use a different warfarin sodium tablet formulation in each one of the phases 2, 3 and 4: either the branded Marevan® (União Química/Farmoquímica, Brasil), or two generic drugs, (manufactured by União Química Farmacêutica Nacional S/A or Laboratório Teuto Brasileiro S/A, Brasil) purchased from Brazilian drugstores (not directly from the manufacturer). Every patient will use each one of the three formulations in a previously determined sequence (A, B, C, D, E or F) to which he was assigned by randomization.