Co‐Prescribing of Potentially Interacting Drugs during Warfarin Therapy – A Population‐Based Register Study

We analysed the occurrence of co‐prescribing of potentially interacting drugs during warfarin therapy in the community‐dwelling population of Finland. We identified drugs having interaction potential with warfarin using the Swedish Finnish IN teraction X‐referencing drug–drug interaction database ( SFINX ) and obtained data on drug purchases from the nationwide Prescription Register. We defined warfarin users as persons purchasing warfarin in 2010 (n = 148,536) and followed them from their first prescription in 2010 until the end of the calendar year. Co‐prescribing was defined as at least 1‐day overlap between warfarin and interacting drug episodes. In addition, we identified persons who initiated warfarin therapy between 1 January 2007 and 30 September 2010 (n = 110,299) and followed these incident users for a 3‐month period since warfarin initiation. Overall, 74.4% of warfarin users were co‐prescribed interacting drugs. Co‐prescribing covered 46.4% of the total person‐years of warfarin exposure. Interacting drugs that should be avoided with warfarin were co‐prescribed for 13.4% of warfarin users. The majority of the co‐prescriptions were for drugs that are not contraindicated during warfarin therapy but require special consideration. Among incident users, 57.1% purchased potentially interacting drugs during the 3‐month period after initiation, while 9.0% purchased interacting drugs that should be avoided with warfarin. To conclude, the occurrence of co‐prescribing of potentially interacting drugs was high during warfarin therapy. Our findings highlight the importance of close monitoring of warfarin therapy and the need for further studies on the clinical consequences of co‐prescribing of interacting drugs with warfarin.