Acotiamide hydrochloride hydrate (acotiamide-HH) has been newly developed as an indication for functional dyspepsia, which is characterized by digestive symptoms such as postprandial fullness, abdominal bloating, or early satiation, and is now being prescribed in Japan. As part of a safety assessment, 2-year long-term carcinogenicity studies using rats and mice were conducted. In the mouse carcinogenicity study, no evidence of carcinogenicity was obtained, even in the high-dose-treated group (up to 2000 mg/kg/day). In the rat carcinogenicity study, acotiamide-HH was administered at 200, 600, and 2000 mg/kg/day. Detailed histopathological examination revealed that the incidence of endometrial adenocarcinoma significantly increased in the 600 mg/kg/day treated group. There was no trend of this incidence and no accompanying increase in pre-neoplastic lesions or related histological changes in the genital tissues, suggesting the absence of abnormalities in the sexual endocrine system. Results of genotoxicity and reproductive/developmental studies showed that acotiamide-HH is a non-genotoxic substance and did not affect sexual balance. Acotiamide-HH did not induce an estrogen-dominant hormonal imbalance that could cause the incidence of uterine cancer and did not have initiation activity. Therefore, the proliferation of endometrial adenocarcinoma in this middle dose group in the rat carcinogenesis study was considered an accidental event of naturally occurring tumors. However, the incidence of endometrial adenocarcinoma in this group deviated from the background data collected in the same laboratory during the study period. Therefore, it is considered necessary to conduct another pre-clinical study in order to obtain data that would dispel any concerns of safety.