Rebamipide, a cytoprotective agent, has been suspected to attenuate oral mucositis through anti-inflammatory potentials and induction of endogenous prostaglandin synthesis. This prospective study was designed to assess the clinical efficacy of rebamipide gargle against oral mucositis, which is induced by fluoropyrimidines in patients with stomach and colorectal cancer. We first conducted a pilot study on gargle flavors, because the solution in this agent has a strong and bitter after taste. Nine kinds of flavors were prepared, and six characteristics were evaluated by ten volunteers: sourness, bitterness, sweetness, remain, after taste, and hard to drink. We determined the contents of rebamipide using HPLC, which showed stability in an acidic condition. Finally, we decided that 100% Pokka Lemon should be used as the flavor of the rebamipide solution. A clinical study was then started to compare the preventive effects rebamipide gargle and placebo have on stomatitis, quality of life (QOL), and the therapeutic effects of chemotherapy.