Potential drug-drug interaction between duloxetine and acenocoumarol in a patient with Alzheimer's disease

Background: Recent evidence suggests that duloxetine may increase the effect of warfarin, thereby increasing the possibility of bleeding. However, a MEDLINE search for articles published between 1980 and May 2007 (terms: duloxetine, anticoagulants, acenocoumarol, and interaction; no language restriction) did not yield any reports of an interaction between concomitant use of duloxetine and acenocoumarol.Objective: The aim of this study was to describe a potential drug-drug interaction between duloxetine and acenocoumarol in a patient with Alzheimer's disease. The possible mechanism of this potential interaction is examined.Case summary: This report presents the case of a 63-year-old white female patient weighing 64 kg receiving oral treatment with the synthetic coumarin anticoagulant acenocoumarol (mean dosage, 9 mg/wk) for 8 years who experienced a persistent (3-week) decrease in international normalized ratio (INR) to 1.49 after taking a single 60-mg dose of duloxetine. Three weeks after discontinuation of duloxetine, the INR returned to the previous baseline level (2.58).Conclusions: This case report suggests a possible drug-drug interaction between duloxetine and acenocoumarol. Duloxetine should be administered with caution in patients receiving acenocoumarol therapy.