In recent years, overdoses related to over-the-counter medicines have become a serious social problem in Japan. As some medicines have been designated as abuse-prone and sales regulations have been strengthened, there is concern that these medicines might be obtained via personal importation. This study focused on dextromethorphan and diphenhydramine, which have been reported to be misused despite not being designated as abuse-prone medicines in Japan. This study investigated public health concerns related to their personal importation to provide evidence to prevent overdoses. Between July 31 and September 8, 2023, products containing these substances were purchased from personal import agency websites. Website information at the time of purchase was reviewed, and the products were examined for appearance, authenticity, and active pharmaceutical ingredient content. In total, 3 dextromethorphan products and 21 diphenhydramine products, all of which were unapproved medicines in Japan, were obtained. Although some products appeared to be prescription-only based on labeling, no prescriptions or identity verification was required, and quantities equivalent to a 1–2-month supply were easily obtained. Most products were also cheaper per tablet than domestically marketed medicines. Appearance inspections revealed inappropriate labeling, damaged tablets, and contaminated packaging. Manufacturers did not respond to authenticity inquiries. The active pharmaceutical ingredient content was approximately 100% of the labeled amount in all samples. Although no clear counterfeit items were identified, substandard-quality medicines were detected. These findings indicate that personal importation should be considered when developing measures to prevent inappropriate over-the-counter medicine use and related health risks.