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Adherence and Concomitant Medication Use among Patients on Warfarin Therapy: Insight from a Large Pharmacy Dispensing Database in Japan

  • Takeuchi Masato
    Department of Pharmacoepidemiology, Graduate School of Medicine and Public Health, Kyoto University
  • Nakano Sayuri
    Department of Pharmacoepidemiology, Graduate School of Medicine and Public Health, Kyoto University
  • Tanaka-Mizuno Sachiko
    Department of Pharmacoepidemiology, Graduate School of Medicine and Public Health, Kyoto University Department of Medical Statistics, Shiga University of Medical Science
  • Nishiyama Chika
    Department of Pharmacoepidemiology, Graduate School of Medicine and Public Health, Kyoto University Department of Critical Care Nursing, Graduate School of Human Health Science, Kyoto University
  • Doi Yuko
    Ain Holdings Inc.
  • Arai Masaru
    Kraft Inc.
  • Fujii Yosuke
    Ain Holdings Inc.
  • Kawakami Koji
    Department of Pharmacoepidemiology, Graduate School of Medicine and Public Health, Kyoto University

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Abstract

<p>Warfarin is a drug used for anticoagulation management, with a narrow therapeutic range and multiple drug–drug interactions. Adherence and proper use of concomitant medication are thus fundamental to the efficacy and safety of warfarin therapy. In 2012, we retrospectively analyzed data from three large-scale pharmacy chains in Japan. We included all adults (≥ 20 years old) with at least one record of warfarin dispensation. We examined patient demographic data, adherence as measured by medication possession ratio (MPR), and co-dispensation focusing on the number of concomitant dispensations and concurrent use of medications that increase bleeding risk. Thresholds of underadherence and overadherence were set at <0.9 and >1.1, considering the narrow therapeutic window. We reviewed 443007 warfarin dispensation records of 71340 individuals (median age, 73 years; 62% male). The MPR was 1.0 (interquartile range: 0.96–1.0), and underadherence and overadherence was found in 16.3 and 1.9% of individuals, respectively. The median number of co-dispensed drugs was eight at each pharmacy encounter, which did not differ by age group. Drugs associated with a high bleeding risk were dispensed in 40.0% of encounters and accounted for 16.4% of all co-dispensed drugs. In summary, we found optimal overall adherence, as assessed by MPR, among our Japanese study population, even when defining a strict cut-off value. However, polypharmacy was common in all age groups and medications with a high bleeding risk profile were often co-dispensed with warfarin. Future research addressing how these dispensation patterns affect patient outcome is warranted.</p>

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