The purpose of this randomized controlled trial is to investigate the non-inferiority, and possible superiority, of a high-dose home-based tDCS protocol compared with a conventional home-based protocol, and to assess its cost-effectiveness, in patients with fibromyalgia.

As complementary goals, we aim to assess the predictive value of baseline EEG for clinical response to high-dose home-based tDCS treatment; and to describe the effectiveness of a high-dose home-based protocol applied to the motor cortex (M1) versus the dorsolateral prefrontal cortex (DLFPC) in patients with fibromyalgia.

Inclusion Criteria:

• Patients who meet the diagnostic criteria described by the American College of Rheumatology (Wolfe et al., 2016):
• Widespread Pain Index (WPI) ≥7 and Symptom Severity Scale (SSS) score ≥5 or a WPI score of 4-6 and SSS ≥9
• Presence of widespread pain, defined as pain in at least 4 of 5 regions. Jaw, chest, and abdominal pain are not included in the definition of widespread pain.
• Symptoms have been generally present for at least 3 months.
• The diagnosis of fibromyalgia (FM) is valid regardless of other diagnoses. The diagnosis of FM does not exclude the presence of other clinically significant diseases.
• Patients with a stable prescription (or lack thereof) for antidepressant/pharmacological medication and who agree to continue it throughout the study.
• Demonstrate the ability to properly administer home-based tDCS independently or with the assistance of a caregiver.
• Have access to an electronic device with a camera (mobile phone or computer) to allow for monitoring of the intervention and communication with the participant.
• Have the capacity and willingness to commit to the study team for the completion of all phases of the study.
• Volunteer to participate and sign the specific informed consent form for this study.

Exclusion Criteria:

• Presenting with immune system disorders or comorbidities that explain the main symptoms of fibromyalgia: rheumatoid arthritis, lupus, autoimmune, neurological, and oncological disorders.
• Presenting with any uncompensated clinical condition such as ischemic heart disease, kidney disease, or liver disease.
• Presenting with dermatological conditions, such as allergic skin reactions at the electrode sites, psoriasis, etc.
• Any exclusion criteria established by clinical guidelines on non-invasive brain stimulation (Woods et al., 2016):
• Metallic implants or head injuries, any electronic device such as cochlear implants or cardiac pacemakers.
• Brain stimulation within the last 6 months.
• A clinical or family history of epilepsy.
• Having any structural lesion (for example, any structural neurological condition or more subcortical lesions than would be expected for their age, or having suffered a stroke affecting the stimulated area or connected areas) or any other clinically significant abnormality that could affect safety, participation in the study, or confound the interpretation of the study results, as determined by the investigator.
• History of drug or alcohol abuse during the study or in the 3 months prior (except for nicotine).
• Changes in drug treatment in the month prior to starting the trial.
• Awaiting trial or litigation during the trial.
• Pregnancy

In accordance with the recommendations of the International Committee of Medical Journal Editors, the de-identified individual participant data (IPD) that underlie the results reported in this study will be shared. The data will become accessible one year after the publication of the primary results and will remain available for five years. Access will be provided to researchers who inquire and agree to a data-use agreement through Ionclinics (investigacion@ionclinics.com/ensayos@ionclinics.com). The data will be de-identified and shared in accordance with applicable regulations and the informed consent provided by the participants.