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Vitamin B12 deficiency usually presents with hematological or neuropsychiatric manifestations. The common clinical presentations include pernicious anemia, myelopathy (Subacute combined degeneration of the cord), myeloneuropathy, optic atrophy, dementia and psychosis. An uncommon manifestation of Vitamin B12 deficiency-acute onset mania-in a 25-years old man that responded to treatment is reported. Relevant literature is also reviewed.