ABSTRACT
<b>Background: </b>Elderly patients are considered to be at risk of developing adverse drug events (ADEs) because they tend to receive a greater number of medications. The purpose of our study is to determine the prevalence of ADEs related to polypharmacy and causative admissions of patients in Japanese acute care hospitalization.<br><b>Methods: </b>In retrospective cohort study, we analyzed 700 consecutive elderly patients admitted to the department of medicine of a Japanese community hospital in 2011. ADEs were defined by World Health Organization–Uppsala Monitoring Centre criteria. Polypharmacy was defined as five or more medications.<br><b>Results: </b>The mean age was 79.5 years (men, 54%). The mean number of medications was 6.36 +/− 4.15 (maximum, 26). Polypharmacy was observed in 63% of cases. ADEs were identified in 4.9% (95% CI, 3.5–6.7%). The mean numbers of medications among patients with ADEs and those without ADEs were 9.3 +/− 3.4 and 6.2 +/− 4.1, respectively. A greater number of medications was significantly associated with ADEs (p < 0.001). Polypharmacy was identified in 91% of patients with ADEs, while it was noted in 62% of patients without ADEs (p = 0.001). Using logistic regression analysis, polypharmacy was significantly associated with ADEs (Odds ratio 5.89, 95% CI 1.74–19.9). The highest number of ADEs were identified among patients on antiplatelets or anticoagulants (n = 8), followed by benzodiazepines and NSAIDs (n = 4 for both). The most common ADEs were gastrointestinal bleeding, nausea and congestive heart failure.<br><b>Conclusion: </b>ADEs complicated to about 5% of acute care elderly hospitalizations in Japan. Polypharmacy was significantly associated with these ADEs.