Increased warfarin doses and decreased international normalized ratio response after nationwide generic switching.

Objective: Our objective was to examine possible changes in the effectiveness of warfarin after a nationwide generic substitution of formulations in 1998.

Methods: In the computerized records of a health maintenance organization database, we identified 975 patients who took warfarin continuously over two 6-month periods, before (period 1) and after (period 2) the generic switch. In this sample we performed a retrospective, between-period paired comparison of warfarin doses dispensed and international normalized ratio (INR) levels maintained, as well as of the apparent warfarin sensitivity index (calculated as INR/Warfarin dose [in milligrams per day]).