[B12 deficiency] ".. can also occur in strict vegetarians (vegans) if they fail to obtain B12 from fortified plant foods, like meat and milk substitutes, or supplements.
A less dramatic, but more common symptom of early B12 deficiency is depression, typically of the listless, mentally foggy kind. In the 1950s, one such woman had so convincing a case of endogenous Depression that shock therapy was vainly administered. Four years later, her slow-onset B12 deficiency was finally diagnosed. A few days and a few shots of B12 later, "she showed a dramatic clinical improvement and came to life again," her doctor T. N. Fraser reported. Another couple of months, and "she looked the picture of health."
Because pernicious anemia is a highly age-related disease, most doctors today are alert for it in elderly patients with neuropsychiatric symptoms. But studies suggest any depressed person has a 10 to 30 percent chance of being B12-shy, usually without pernicious anemia as the cause. That deficiency, according to Cees van Tiggelen et al., "has profound effects on several neurotransmitter systems and results in significantly reduced norepinephrine levels in the brain." Norepinephrine is one of the brain's most important good-mood neurotransmitters. "