Forty-eight patients (eight in each group) is the minimum necessary number of individuals to detect a clinically significant fluctuation of 0.49 in the mean INR, with a 90% power at a 5% level of significance, if one were present. This sample size was estimated considering the Pocock's statistical method for quantitative outcomes and an achieved mean INR and standard deviation with the branded warfarin of 2.45 and 0.29 respectively [update: later we considered a more conservative standard deviation of 0.34, which resulted in a sample size of 11 patients per group]. To compensate possible withdrawals or exclusions of patients we plan to recruit 60 patients.