Probable Interaction Between Tolterodine and Warfarin
Probable Interaction Between Tolterodine and Warfarin
A 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.
Overactive bladder is characterized by involuntary and unpredictable contractions of the detrusor muscle during the filling phase of the micturition cycle, causing an increased micturition frequency and a strong urge to void. Urinary incontinence occurs when the urge to void cannot be suppressed. Tolterodine is a muscarinic receptor antagonist for the treatment of overactive bladder. It has selectivity for the bladder over the salivary glands. Extended-release tolterodine 4 mg/day is effective and well tolerated in the treatment of overactive bladder.
We report the case of a patient who started tolterodine for overactive bladder while she was concurrently taking warfarin. Although the dosage of warfarin was reduced when tolterodine was prescribed, the patient's international normalized ratio (INR) subsequently increased. To my knowledge, only one other report describes an interaction between tolterodine and warfarin. Based on that report and our patient's experience, a potential interaction between these drugs exists, and the dosage of warfarin may need to be decreased in patients who begin tolterodine therapy.
Abstract and Introduction
Abstract
A 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.
Introduction
Overactive bladder is characterized by involuntary and unpredictable contractions of the detrusor muscle during the filling phase of the micturition cycle, causing an increased micturition frequency and a strong urge to void. Urinary incontinence occurs when the urge to void cannot be suppressed. Tolterodine is a muscarinic receptor antagonist for the treatment of overactive bladder. It has selectivity for the bladder over the salivary glands. Extended-release tolterodine 4 mg/day is effective and well tolerated in the treatment of overactive bladder.
We report the case of a patient who started tolterodine for overactive bladder while she was concurrently taking warfarin. Although the dosage of warfarin was reduced when tolterodine was prescribed, the patient's international normalized ratio (INR) subsequently increased. To my knowledge, only one other report describes an interaction between tolterodine and warfarin. Based on that report and our patient's experience, a potential interaction between these drugs exists, and the dosage of warfarin may need to be decreased in patients who begin tolterodine therapy.
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