Since patients included in the study will already be using warfarin and it is not ethically acceptable to interrupt this treatment, we planned a run-in period equivalent to seven warfarin half-lives, long enough to washout any previous warfarin treatment. Then, in this first phase, patients will start treatment with the same warfarin formulation that will be used in the second phase thus avoiding the carry-over effect. This period will also be used to select patients compliant with treatment, accepting those that during the run-in have at least one of the three (including the baseline) INR results within 2.0 and 3.0 and a difference of INR results at the 3th and 4th week ≤ ±0.8.