A Phase 2 Trial of Rebamipide Liquid to Determine the Effective Dose for Prevention of Chemoradiotherapy-induced Oral Mucositis in Patients With Head and Neck Cancer

Inclusion Criteria: * Head and neck cancer patients scheduled for definitive or postoperative chemoradiotherapy. * Patients with a histopathological diagnosis of head and neck cancer and primary tumor in one of the following regions. 1. Definitive therapy: nasopharynx, oropharynx, hypopharynx, or larynx 2. Postoperative therapy: oral cavity, oropharynx, hypopharynx, or larynx * Patients with no history of chemotherapy, radiotherapy, or chemoradiotherapy for head and neck cancer * Patients with an Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1 * Patients who are able to hold fluid in the mouth * Patients who are able to swallow the investigational medicinal product (IMP) * Patients expected to survive for at least 3 months * Patients who have given written informed consent in person * Patients who can stay at or visit the hospital for scheduled examinations and observations * Patients who are able to take contraceptive measures to avoid pregnancy of the patient or their partner from the time of informed consent until 4 weeks after completion of IMP administration Exclusion Criteria: * Patients with primary malignant tumors other than head and neck cancer. * Patients with symptomatic viral, bacterial, or fungal infection * Patients with serious renal impairment * Patients with distant metastasis * Patients with severe complications (uncontrolled cardiac disease, diabetes, hypertension, etc) * Patients with any of the following laboratory test results: 1. Neutrophil count: \<1500 L 2. Platelet count: \<75000 L 3. Hemoglobin: \<10.0 g/L 4. Aspartate aminotransferase (AST): \>3 times the upper limit of the reference value at the trial site 5. Alanine aminotransferase (ALT): \>3 times the upper limit of the reference value at the trial site 6. Serum bilirubin: \>1.5 times the upper limit of the reference value at the trial site 7. Serum albumin: \<3.0 g/dL 8. Serum creatinine: \>1.5 the upper limit of the reference value at the trial site 9. Creatinine clearance : \<30 mL/min * Patients complicated with autoimmune disease * Patients requiring continuous systemic administration of glucocorticoid * Female patients who are pregnant or lactating, who may possibly be pregnant, or who wish to become pregnant * Patients who have participated in any other clinical trial within 4 weeks prior to initiation of chemoradiotherapy * Patients who have a history of drug allergy to rebamipide, cisplatin, or other platinum compounds * Patients who are otherwise judged by the investigator or sub-investigator to be inappropriate for inclusion in the trial