A deficiency leads to pernicious anemia, characterized by weakness, shortness of breath, and a few other symptoms related to malfunction of the nervous system such as a lack of coordination.
Pernicious anemia is a fairly rare, but very serious condition.
Severe B12 deficiency leads to irreversible neurologic damage. Absorption of vitamin B12 in the intestine is dependent on a substance made in the stomach called intrinsic factor. Production of the compound often decreases in the elderly, who are at risk for B12 deficiency and anemia. People with B12 deficiency are usually treated with the injectable form of the vitamin to bypass the intestinal absorption. However, there are two studies in the medical literature which prove that B12 can be absorbed through the lining of the nose.