Probable interaction between tolterodine and warfarin.

53-year-old woman with a history of deep vein thrombosis and pulmonary embolism was receiving warfarin to prevent thromboembolic complications; her international normalized ratio (INR) had been stable for 1 month. Extended-release tolterodine 4 mg/day was then prescribed to manage overactive bladder. On her next anticoagulation clinic visit, the patient's INR had increased, although the dosage of warfarin had been reduced when the tolterodine had been prescribed. Due to the absence of other contributing factors and the temporal relationship between tolterodine and prolonged INR, the event was determined to be a probable drug interaction. When patients are prescribed tolterodine and warfarin concurrently, clinicians should monitor INR carefully, and a reduction in warfarin dosage may be required.