Inclusion Criteria:  Diagnosis of Major Depressive Disorder according to DSM-5 criteria Treatment-resistant status, assessed by clinical interview, after four weeks after SSRI therapy prescription as per NICE guidelines (National Institute for Health and Care Excellence, 2009) Age between 18 and 65 years Signature of informed consent  Exclusion Criteria:  Presence of intellectual disability or illiteracy Diagnosis of a disorder belonging to the spectrum of schizophrenia or other psychotic disorders according to DSM-5 Diagnosis of a disorder belonging to the bipolar spectrum according to DSM-5 Concomitant intake of therapy with other antidepressants, antipsychotics or mood stabilizers at the time of evaluation or in the previous 4 weeks State of pregnancy