In Japan, stomach and colorectal cancers are the most frequent causes of cancer-related death despite dramatic advances in diagnosis and surgical treatment. Chemotherapy plays an important role in the treatment of these cancers, and fluoropyrimidines, such as 5-fluorouracil (5-FU) and S-1, are widely used as key drugs . Furthermore, the recent progress in chemotherapy has extended the overall survival of patients with these cancers , in whom fluoropyrimidines are sometimes used for periods longer than a year. Oral mucositis is one of the most common symptomatic complications associated with the use of fluoropyrimidines . Severe oral mucositis can cause intolerable pain and increase the risk of systemic infections, necessitating dose reduction and discontinuation of cancer chemotherapy. Moreover, this adverse effect may have an impact on patient nutrition and quality of life (QOL) , and therefore, an effective and prophylactic intervention to alleviate this complication is needed.