Formulary Considerations Related to Warfarin Interchangeability

INTRODUCTION For years, Coumadin (DuPont Pharma Inc., Mississauga, Ontario) was the only warfarin product marketed in Canada. Recently, 2 additional warfarin products have received approval for sale in Canada: Taro-Warfarin (Taro Pharmaceuticals Inc., Bramalea, Ontario) and Apo-Warfarin (Apotex Inc., Weston, Ontario). Warfarin is a drug with a narrow therapeutic range, as defined by Health Canada1: its ratio of minimum toxic concentration to median effective concentration is less than or equal to 2. In addition, warfarin has an unpredictable dose response; therefore, close therapeutic monitoring is required for optimal antithrombotic effect with minimal bleeding complications. Because minor dose changes can result in clinically significant changes in the international normalized ratio (INR), the issue of equivalence between brand name and generic warfarin products requires careful evaluation.2 Table 1 presents several characteristics used in assessing the degree of equivalence between the various warfarin formulations now available in Canada. These variables were used in deciding on the warfarin product to be included on the formulary at the authors’ hospital.