 The most vulnerable period for thrombosis and hemorrhage due to warfarin is during the initiation phase of anticoagulation, when optimal dosing is least certain. Adverse event rates are highest during this vulnerable period, which is generally considered to persist for at least 3 months. Patients who have not been previously exposed to warfarin are most vulnerable because there is no individual history to guide the clinician on an optimal initiation dose for those particular patients. 