Since 2011, pharmaceutical companies in Japan have been required to issue two types of documents regarding severe adverse drug reactions reported post-marketing, namely the Rapid Safety Communication Materials for Patients and the Related Materials. However, the adequacy of these documents has not yet been systematically assessed. The aim of this study was to evaluate the adequacy of these two types of materials. The Rapid Safety Communications for Patients were obtained from the Pharmaceuticals and Medical Devices Agency (PMDA) website. The Related Materials were obtained from pharmaceutical companies or the PMDA website. Three assessors independently scored the Rapid Safety Communication for Patients and the Related Materials using the Centers for Disease Control and Prevention Clear Communication Index (CCI). In addition, the contents and descriptions of the materials were analyzed. In total, 13 materials for seven drugs were assessed. Almost all materials contained the "main message" and "call to action". However, the average CCI scores for the Rapid Safety Communication for Patients and Related Materials for Patients were 68.8 and 74.3 (out of 100), respectively. Further, none of the evaluated materials were scored above the CCI threshold score (i.e., ≥ 90%). Descriptions regarding "language", "state of science", and "risk" were not adequate. In particular, the terminology used in materials seemed difficult for patients to understand. In conclusion, the Japanese Rapid Communication Materials for Patients require improvement. Furthermore, a system for evaluating these materials prior to publication should be established.