An independent investigator (initials TFCP) will use a computer random number generator to allocate patients to one of the six sequences of treatment [update: we opted to apply randomization through numbered, opaque, sealed envelopes in order to facilitate allocation concealment]. The principal investigator (initials CGF) will separate, repack and dispense drugs for patients in opaque drug containers of identical appearance [update: the step of repacking the drugs in opaque containers was made by two investigators external to the study, BR and MFST, instead of the principal investigator]. This process will be based on the initials of the patient and the alphanumeric code, maintaining the allocation concealment since TFCP will be responsible for assigning to the repacked drugs the patient initials and the alphanumeric code correlated with the formulation of warfarin utilized. TFCP will also be accountable to conceal the allocation until the statistical analysis at the end of the study [update: BR and MFST created the coding and maintained its concealment]. Due to the features of the drug containers, physicians and the principal investigator (that assess patients and collect outcomes) will be blinded. Patients may not be blinded depending on the appearance of the manufactured tablets, but the main outcome will be a laboratory result (INR).