The purpose of this multicentric randomized controlled trial is to compare the effectiveness and safety of home-based tDCS for the treatment of major depression versus tDCS treatment in a healthcare centre, and explore the effects of an accelerated home-tDCS protocol.

The change in depression index at the end of treatment, measured with MADRS, will be the primary outcome.

Inclusion Criteria:

• Spanish or Catalan speakers of both sexes between the ages of 18 and 65.
• Patients diagnosed with major depression, based on the criteria in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5). Specifically, patients will be included who meet the criteria for a major depressive episode with a longitudinal diagnosis of major depressive disorder, single or recurrent episode; or bipolar disorder type 1 or 2. Patients must obtain a moderate or higher score, therefore we establish a lower limit of 15 points on the Montgomery-Åsberg Depression Rating Scale (MADRS) (Alonzo et al., 2019).
• Patients receiving or not receiving pharmacological treatment for depression will be included, and this will be recorded as an additional variable.
• Have the ability and willingness to commit to the study team for supervision of the intervention sessions and close monitoring of safety.
• Demonstrate the acquisition of the ability to properly apply home tDCS independently or with the help of a companion.

Exclusion Criteria:

• Patients with psychotic, schizoaffective, or personality disorders (both cluster A and B).
• Patients with a history of neurological disease, intellectual disability, or cognitive impairment (inability to understand instructions or operate equipment).
• Any exclusion criteria established by clinical guidelines on non-invasive brain stimulation (Woods et al., 2016): metal implants or head injuries, any electronic devices such as cochlear implants or cardiac pacemakers. Brain stimulation in the last 6 months. Clinical or family history of epilepsy.
• Patients with dermatological problems, such as an allergic skin reaction at the electrode site.
• High risk of suicide. Assessed through an interview with a psychiatrist and the use of the Spanish-validated Columbia Suicide Risk Scale (C-SSRS) (Al-Halabí et al., 2016).
• Drug or alcohol abuse during the study or in the previous 3 months (except for nicotine).
• Changes in pharmacological or non-pharmacological treatment (such as structured psychotherapy) during the study or in the 3 months prior to starting 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.

Although it is impossible to blind participants due to the nature of the study, the evaluator and statistician will be blinded.