A long list of psychiatrically inclined illnesses or symptoms, especially some cases of mood disorder, dementia, paranoid psychoses, violent behavior and fatigue, have been documented to be caused by vitamin-B12 deficiency, among other causes. The author uses reputably published literature--and extrapolations from it--to show that these conditions are possibly more commonly caused by B12 deficiency than is currently generally accepted, mostly because of a lack of appreciation of the lowest serum-B12 level that is necessary to protect against the cerebral manifestations of this deficiency. After surveying the whole area of psychiatry and nutritional deficiencies in general, the author deals with the role of vitamin-B12 in mood disorders, paranoid psychoses and dementia in more detail. In doing so, he cites some useful conclusions from the literature, including the debunking of several myths about the diagnosis and treatment of brain-B12-deficiency, especially the efficacy of high dose oral treatment and the relative inefficacy of the Schilling's test.