Oral mucositis is the most significant acute toxicity of oral cavity radiotherapy. The available scientific evidence supports its preventive and therapeutic approach with low-level laser; however, this is unsuitable for tumor regions due to the risk of stimulating cellular metabolism. In this sense, an alternative treatment becomes necessary. Based on the preliminary results of the phase I study, a double-blind, randomized phase II study is suggested to evaluate the effectiveness of copaiba-based mouthwash in the prevention of oral mucositis in patients undergoing radiotherapy for oral cavity tumors. Patients will be randomized into 2 groups: A (copaíba) and B (placebo) and will use the mouthwash 4x/day. Each group will have 20 patients and be blind to the group in which they are included. They will be evaluated daily by a dental surgeon about oral mucositis, pain in the oral cavity and oropharynx, and dysphagia and will undergo daily laser therapy sessions, until the end of radiotherapy.