Background: Obsessive–compulsive disorder (OCD) is a mental disorder in which a person has certain repeated thoughts (called "obsessions") or feels the need to perform repeated behaviors (called "compulsions") to a level that causes distress or impairs general functioning. Amounts of evidence show that folate and vitamin B12 are important nutrient elements involved in the physiological and pathological functions of the neuropsychiatric system. The incidence of neuropsychiatric findings in patients with vitamin B12 deficiency has been investigated worldwide. More recently, folate and vitamin B12 have been reported in a large amount of literature as treatments or supplements for the treatment of psychiatry disease.