A deficiency of vitamin B12 due to the absence of the vitamin or decreased ability of the body to absorb the vitamin. A deficiency can result from diets that exclude all animal products and is also associated with various autoimmune disorders, especially those involving the thyroid, parathyroid, and adrenal glands. This deficiency can also develop if stomach acid is not produced in sufficient amounts. Other conditions that can inhibit the body’s absorption of vitamin Bi2 include celiac disease, Crohn disease, Whipple disease, intestinal worms, structural defects of the intestinal system, surgical removal of the stomach, poisoning with corrosive substances, and tuberculosis. A vitamin B12 deficiency can cause pernicious anemia, a disease in which red blood cells are abnormally formed. Treatment of pernicious anemia is with lifelong injections of vitamin Bi2 or very high doses of oral vitamin Bi2. Other forms of B12 deficiency that do not involve a lack of intrinsic factor may be treated with oral vitamin B12 supplements.