A nutritional condition produced by a deficiency of VITAMIN B 12 in the diet, characterized by megaloblastic anemia. Since vitamin B 12 is not present in plants, humans have obtained their supply from animal products, from multivitamin supplements in the form of pills, and as additives to food preparations. A wide variety of neuropsychiatric abnormalities is also seen in vitamin b 12 deficiency and appears to be due to an undefined defect involving myelin synthesis. (From Cecil Textbook of medicine, 19th ed, p848)