Objective: The delivery of medicines by the postal service from pharmacies to patients' homes has increased to address medicine supply shortages. However, owing to recent abnormal weather conditions, medicines may be delivered in harsh environments exceeding 40℃. Therefore, this study aimed to measure the temperature at which medicines were mailed in the summer and investigate whether the interview form contained evidence that could guarantee the quality of the medicines mailed under those temperature conditions.Methods: Between August 5 and September 11, 2024, medicines were mailed from 15 postbox locations using a service provided by the Japan Post. The mailing environment was assessed via mean kinetic temperature (MKT) with reference to the United States Pharmacopeia (USP). The medicines in shortage were identified based on information published by the Japan Pharmaceutical Manufacturers Association.Results: Approximately 87.5% of the medicines mailed had an MKT above 25℃ (maximum 28.0℃), which deviates from the USP standard. Some medicines temporarily reached a maximum temperature of 47.5℃. Moreover, the results of the stress tests at temperatures above 40℃ were not listed in the interview form for 78.3% of the medicines that were likely to be mailed due to inventory shortages.Conclusion: To guarantee the medicines quality when mailed in the summer, it is important to consider appropriate measures to protect medicines from excessive heat, even medicines stored at room temperature. Furthermore, it is important to assess the medicine stability by referring to the stress tests (50℃) results listed on the interview forms for the product in question and the original and longlisted drugs. To resolve dissatisfaction between pharmacies and patients due to supply instability, it is necessary to actively publish the results of stress tests on interview forms and make appropriate efforts.