WHAT IS KNOWN AND OBJECTIVE: Polypharmacy is associated with an increased risk of adverse drug reactions (ADRs) and drug interactions, decreased adherence to medication and increased medical cost. Recently, polypharmacy has become a major problem in medical care in Japan as a result of the increase in the ageing population. The purpose of this study was to investigate the current situation of polypharmacy and the association between polypharmacy and adverse events. METHODS: A retrospective data analysis was performed using two different real-world data from 2007 to 2015 in Japan. The Japanese Adverse Drug Event Report (JADER), a public spontaneous adverse drug reaction database constructed by the Pharmaceuticals and Medical Devices Agency (PMDA), and a large prescription database constructed by a database vendor (Japan Medical Information Research Institute, Inc Japan [JMIRI]) were analysed. Trends of polypharmacy during the study period were investigated. RESULTS AND DISCUSSION: The mean number of drugs per report in the JADER database and per prescription in the JMIRI databases during the study period ranged from 4.8 to 5.6 and 3.5 to 3.7, respectively. The mean number of drugs increased with age in both the JADER and JMIRI databases, and the peak of the mean number of drugs was at 80-89 years (5.74 drugs) in the JADER database and at 90-99 years (4.97 drugs) in the JMIRI database. WHAT IS NEW AND CONCLUSIONS: The number of drugs increased until age 90 years or more, even though adverse events are more likely to occur after the age of 80 in Japan. Therefore, polypharmacy in the elderly should be focused on the patients aged ≥80 years rather than patients aged ≥65 years from the viewpoint of the prevention of adverse events.