• V J Colucci and M P Rivey.
• Pharmacy Practice, School of Pharmacy, University of Montana, Missoula 59812, USA. colucci@selway.umt.edu
• Ann Pharmacother. 1999 Nov 1; 33 (11): 1173-6.

ObjectiveTo report two cases of warfarin therapy in which the addition of tolterodine resulted in prolonged international normalized ratios (INRs).Case SummaryTwo patients, each receiving warfarin for stroke prophylaxis in association with chronic atrial fibrillation, developed adverse effects after the initiation of tolterodine for urinary disorders. Other medications for concurrent medical diagnoses had remained unchanged. One patient had an episode of prostatitis, which was treated with levofloxacin immediately prior to tolterodine initiation. The warfarin dosage had remained constant for many weeks in both patients prior to and during the tolterodine trials. In each patient, the initiation of tolterodine was associated with a significant increase in the patient's INR measured 10-14 days later. Thus, tolterodine was ineffective in both patients and was discontinued one to two days before the elevated INRs were determined during routine clinic visits. INRs determined approximately two weeks after tolterodine was discontinued were similar to those obtained during the period before the use of the drug; the warfarin dosage remained unchanged. Rechallenge with tolterodine was not attempted in either patient.DiscussionSeveral aspects of the reported cases support the validity of a proposed drug interaction when tolterodine is initiated in a patient stabilized on warfarin therapy. The temporal association of the course of tolterodine with an elevated INR, the return to the previous warfarin dose-INR response relationship after tolterodine discontinuation, and the absence of other causes for the elevated INR were factors found in both patients. Possible mechanisms to explain the suggested drug interaction are explored.ConclusionsUntil further data are available, clinicians should be vigilant for a possible drug interaction when tolterodine therapy is initiated in a patient maintained on warfarin therapy.

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