Doxepin Solution for Alleviation of Stubborn Breakthrough Pain Induced by Swallowing in Patients Receiving Radiotherapy for Nasopharyngeal Carcinoma

Inclusion Criteria: 1. Provide informed written consent. 2. Age ≥ 18 years. 3. Histologically confirmed as nasopharyngeal carcinoma, and currently undergoing radical radiotherapy or chemoradiotherapy. 4. Physical examination demonstrating the presence of radiation-induced mucositis in the oral cavity and/or oropharynx. 5. At least 4 (out of 10) patient-reported swallowing-induced pain as measured by the numeric rating scale of pain. 6. Being able to complete the questionnaires independently or with assistance. 7. ECOG Performance Status 0, 1 or 2. Exclusion Criteria: 1. Known allergy to doxepin, tricyclic antidepressants, or any known component of the drug formulation. 2. Use of a tricyclic antidepressant or monoamine oxidase inhibitor within 14 days prior to registration. 3. Current untreated or unhealed oral candidiasis or oral herpes simplex virus infection. 4. Untreated narrow angle glaucoma within 6 weeks prior to registration. 5. Untreated urinary retention within 6 weeks prior to registration. 6. Administration of cryotherapy to prevent oral mucositis within 6 weeks prior to registration. 7. Current serious heart disease or a recent history of myocardial infarction. 8. Current untreated or unresolved conditions like epilepsy, hyperthyroidism, hepatic dysfunction, delirium, and neutropenia. 9. Pregnant or lactating women.