Sumor is a food supplement that combines the main coenzymes of the S-Adenosyl methionine (SAMe) cycle, namely vitamins B6, B12 and folate, with SAMe and betaine. Sumor also contains vitamin C, a molecule that has shown promise in the treatment of depression in experimental models, and selenium, an antioxidant agent whose blood deficiency has been associated with depressive symptoms in some preliminary studies. There are no studies in the literature on the efficacy of this combination in the adjuvant therapy of depression. The purpose of this study is to compare the effects of Sumor in co-therapy with an SSRI antidepressant versus co-therapy with placebo in patients with treatment-resistant Major Depressive Disorder.

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