Prevalence and determinants of unintended double medication of antihypertensive, lipid-lowering, and hypoglycemic drugs in Austria: a nationwide cohort study.

Purpose: Double medication is defined as the unintended overlapping prescription of two identical substances with the same route of administration by two different prescribers to the same patient. Consequences of double medication are reduced patient safety and excess healthcare costs. Based on nationwide prescription data from 2011 covering 97% of Austria's population, we estimated double medication prevalences for treatment of hypertension, hyperlipidemia, and diabetes mellitus.

Methods: We investigated prescriptions of 88 antihypertensive, 16 lipid-lowering and 29 hypoglycemic substances in 7,971,323 persons in 2011. Prevalence of double medication was calculated patientwise (prevalence by patients) and timewise (prevalence by patient-years). Risk factors for double medication were identified by logistic regression.

Results: For antihypertensive, lipid-lowering, and hypoglycemic subtances, overall 15.0% (men: 15.1%, women: 15.0%), 13.1% (13.7%, 12.5%), and 13.0% (13.0%, 13.4%) of patients were doubly medicated, respectively. Corresponding prevalences by patient-years were 1.6%, 2.0%, and 1.2%. Logistic regression confirmed lower age and copayment waiver as independent risk factors of double medication. Furthermore, double medication occurred more often with prescriptions from hospitals or internal medicine specialists compared with general practitioners, as well as in August compared with earlier or later in the calendar year.

Conclusion: While appropriate care or comanagement of patients by internal medicine specialists and general practitioners may explain some of the double prescriptions, our data indicate that unintended double medication is frequent. In Austria, lack of financial incentives of patients to avoid filling duplicate prescriptions explains a considerable fraction of double medication occurrences.