Aim Polypharmacy is an extremely important problem, because it increases the risk of adverse drug reactions. The aim of the current study was to create a clinical medication review tool to detect inappropriate medication use, and assess this new method with elderly Japanese patients. Methods The new method involves optimizing prescription drugs from indications, based on the chronic disease‐anatomical therapeutic class code list. The present study investigated the prevalence of potentially inappropriate medications in 5667 Japanese patients aged ≥65 years with polypharmacy (≥5 drugs) in comparison with the Beers criteria 2012. Results We propose a new method called the Mapping Approach for Pharmacotherapeutic Classifications: (i) identify the chronic disease‐anatomical therapeutic class code assigned to the prescription drugs; (ii) identify the chronic disease‐anatomical therapeutic class code corresponding to the patient's chronic disease; (iii) compare the prescription drug and patient's chronic disease chronic disease‐anatomical therapeutic class codes; and (iv) identify the appropriateness of medication use based on the comparison (appropriate use is defined as matching codes). The mean number of potentially inappropriate medications detected was significantly different between the mapping approach and Beers criteria 2012 (3.1 ± 2.6 vs 0.6 ± 0.8 drugs, respectively; P < 0.001). Conclusions The Mapping Approach for Pharmacotherapeutic Classifications is highly dependent on the chronic condition. Pharmacists should confirm the chronic condition with the treating physician before reducing a patient's medications. We hope this process will further influence prescribing patterns, and decrease the inappropriate use of medications and associated adverse drug reactions in older adults. Geriatr Gerontol Int 2017; ••: ••–••.