We conducted a retrospective study to investigate adverse drug reactions and associated medical costs among elderly individuals that could be avoided if pharmacotherapy was performed in accordance with the Beers Criteria: the Japanese Version (BCJV) and Guidelines for Medical Treatment and Its Safety in the Elderly 2015 (GL2015). Patients aged at least 65 years who were either hospitalized at Gifu Municipal Hospital between October 1 and November 30, 2014 ( n = 1236) or had outpatient examinations at Gifu Municipal Hospital on October 1–2, 2014 ( n = 980) were included in the study. The outcomes measured were usage rates of drugs listed in the BCJV and GL2015, incidence rates of adverse drug reactions, and additional costs incurred per patient due to adverse reactions. Among the inpatients, usage rates of drugs listed in the BCJV and GL2015 were 24.0 and 72.4%, respectively, and adverse reactions to these drugs occurred at rates of 3.0 and 8.2%, respectively. Among the outpatients, while the usage rates were 26.2% (BCJV) and 59.9% (GL2015), the incidence rates of adverse reactions were 4.7% (BCJV) and 3.9% (GL2015). The additional costs incurred due to adverse drug reactions ranged from 12713–163925 yen per patient. Our results demonstrate that appropriate use of drugs based on the BCJV and GL2015 can help prevent adverse reactions; this would reduce the overall medical costs.