Inclusion Criteria:  Patients diagnosed with head and neck cancer Persistent xerostomia after radiotherapy ≥18 years Irradiated with radiotherapy in the major salivary glands (parotid, submandibular and sublingual) Grade 3 for dry mouth in Common Terminology Criteria for Adverse Events, CTCAE (version 5.0) Have completed medical treatment with full response (complete remission) and receive medical clearance for participation. At least one month after radiotherapy completion, to reflect the possible presence of oral mucositis (sores) and/or radiodermatitis (inflammation) that limits adherence to treatment No use of drugs/devices/products (pilocarpine, cevimeline, amifostine, oral devices, humidifiers or herbs) to prevent or treat xerostomia before inclusion in the study, OR constant usage (do not change type and dosage) during 2 months before inclusion in the study  Exclusion Criteria:  Relapse or metastasis Karnofsky activity scale <60 Contraindications to PBM therapy (cardiac arrhythmias, pacemakers, photosensitivity, drugs with photosensitizing action, pregnancy)