Introduction: Burning mouth syndrome (BMS) is a pathology with a low prevalence, affecting between 0.1% and 3.7% of the general population, the ratio between women and men is between 3 and 7:1. It is clinically characterized by a burning pain sensation, recurrent daily for more than 2 hours a day, persistent for more than 3 months and in a way that increases throughout the day and decreases at night, without presenting clinically evident causal lesions on the surface of the oral mucosa, nor changes in clinical sensory tests.

The description of the symptomatology varies according to the patient and may be related especially to psychogenic factors.

Patients report dry mouth/xerostomia and taste alterations, such as metallic or bitter taste. The most frequent location is on the tongue (tip and lateral borders), but it can also include the anterior palate, gingiva and lips.

Since the etiopathogenic factors that produce BMS are not clear, it is difficult to make advances in therapy. The available treatments aim to control the multiple factors related to BMS, thus reducing the symptoms described by patients. Among them, the literature describes low level laser photobiomodulation.

The success of the use of low level lasers for treatments in oral medicine is due to their biomodulatory activity and their ability to penetrate tissues. The analgesic action of laser radiation is due to the inhibition of nociceptive mediators and the release by the Central Nervous System (CNS) of endogenous analgesic substances such as endorphins, which hinders the transmission of the painful stimulus.

It has been verified that low power laser radiation therapy can be effective in reducing symptoms in patients with BMS. Importantly, low power laser therapy is non-invasive, well tolerated by patients.

Objectives: To confirm the hypothesis that the application of low power laser in the areas of oral burning can improve the symptomatology of BMS.

Material and methods: We propose a clinical study in 38 patients with idiopathic BMS, prospective, randomized, double-blind.

The study universe will be constituted by patients attended at the Oral Medicine, Oral Surgery and Implantology Unit of the Faculty of Dentistry of the University of Santiago de Compostela (USC).

The Hospital Anxiety and Depression Scale (HADS), quality of life (OHIP-14) and subjective sensation of dry mouth xerostomia inventory (XI) questionnaires will be applied to all the patients, and the amount of saliva will be quantitatively determined by means of the unstimulated global saliva test (TSG-I) using a millimetric absorbent paper strip, at the beginning, at the end of treatment and at the re-evaluation at two and six months.

Inclusion Criteria:

• Patients with a diagnosis of BMS and symptoms of at least 3 months duration;
• Clinical examination confirms clinically normal mucosa;
• Patients who are not undergoing treatment for BMS;
• Patients who, after being informed about the objectives and procedures of the investigation, agree and sign the informed consent form.

Exclusion Criteria:

• Patients with uncontrolled systemic diseases (ASA III, IV);
• Patients who have received previous radio and/or chemotherapy of the head and neck;
• Patients with secondary BMS, which is due to organic causes, such as biological factors, i.e. the presence of some bacteria or fungi, which have a direct irritant effect on the oral mucosa and are capable of triggering burning symptoms. Such conditions considered as differential diagnosis of primary BMS which are, oral candidiasis, erosive lichen planus, geographic tongue;
• Patients presenting systemic factors such as Sjögren's syndrome, untreated diabetes, or if they use drugs that cause oral burning;
• Patients presenting a VAS score below 3 out of 10.

Removal criteria:

-Patients who for any reason fail to keep follow-up appointments.

The investigators will anonymize and categorize the clinical data of the patients to share the information with the other researchers of the group.

One group will be lasered off and the other group will be lasered on.