What are the psychiatric manifestations of vitamin B12 deficiency?

Psychiatric symptoms attributable to vitamin B12 deficiency have been described for decades. The earlier reports are for the most part in accord with more recent ones, despite being diagnostically less specific in psychiatric and hematologic terms. These symptoms seem to fall into several clinically separate categories: slow cerebration; confusion; memory changes; delirium, with or without hallucinations and/or delusions; depression; acute psychotic states; and (more rarely) reversible manic and schizophreniform states. While there still remain abundant hematologic, psychiatric, neurologic, or nutritional reasons for obtaining a serum vitamin B12 level, its use in the investigation of the etiology of a patient's dementia seems unjustified. However, acute or subacute changes in a demented patient's mental status, specifically a clouding of their consciousness, may make such testing advisable as part of the complete workup of their delirium regardless of a normal hematologic picture.