Bleeding is a common and potentially serious adverse drug reaction associated with warfarin therapy. Warfarin undergoes several drug interactions with frequently used pain medications, which themselves are recognized as an independent risk factor for bleeding. Furthermore, warfarin and pain medications are commonly used by older persons.

This thesis was conducted as three observational studies utilizing data obtained from real-life clinical care. The purpose of this study was to investigate the frequency and clinical consequences of drug interactions in various patient populations and different phases of warfarin therapy. More specifically, the frequency of coprescribing potentially interacting pain medications among warfarin users in outpatient and inpatient settings was determined. Moreover, the clinical consequences of co-administration of potentially interacting pain medications among warfarin users in an inpatient setting were investigated. Finally, the effect of warfarin initiation was examined on the use of potentially interacting pain medication in an outpatient setting and among frail and non-frail inpatients.

These studies demonstrate that co-prescribing of warfarin and potentially interacting pain medications was more common in inpatients than in outpatients. Accordingly, an increased bleeding risk associated with potentially interacting pain medication was observed in inpatients. At the time of warfarin initiation, the interaction frequencies differed between pain medication classes. The use of paracetamol increased in warfarin initiators in both outpatient and inpatients excluding frail inpatients, whereas the opposite was seen for the use of NSAIDs.

In conclusion, the frequencies of warfarin drug interactions differed between pain medication classes, settings of care, and phases of warfarin therapy. The increased risk of bleeding due to potential drug interactions still requires attention when pain medications are prescribed to warfarinized patients.