A vitamin B12 deficiency could be a contributing cause of bursitis, calcium bone spurs, confusion (especially in the elderly), depression (especially in the elderly), dermatitis, easy bruising, elevated homocysteine - increasing the risk of heart disease, fatigue, insomnia, loss of appetite, macrocytic anemia (abnormally enlarged red blood cells), memory loss (especially in the elderly), nausea, peripheral neuropathy (nerve damage), pernicious anemia, poor blood clotting, shortness of breath, skin sensitivity, tongue and mouth irregularities, vomiting and weakness. A substance known as "intrinsic factor" (produced in the lining of the stomach) is needed to absorb vitamin B12. Many people do not produce enough intrinsic factor, particularly people who have had stomach surgery or who have other stomach problems, and research shows that many elderly people do not produce enough intrinsic factor and are frequently found to be vitamin B12 deficient.

Pharmaceutical drugs that can cause a vitamin B12 deficiency include aminoglycosides, cephalosporins, chlorotrianisene, chlortetracycline, cholestyramine resin, cimetidine, colchicine, colestipol, co-trimoxazole, demeclocycline, famotidine, fluoroquinolones, lansoprazole, macrolides, metformin, minocycline, neomycin, nizatidine, omeprazole, oral contraceptives, oxytetracycline, penicillins, phenytoin, potassium chloride, ranitidine bismuth citrate, ranitidine hydrochloride, sulfonamides, tetracyclines, trimethoprim and zidovudine.

Vitamin B12 is produced by microbial synthesis in the digestive tract of animals; it does not occur in fruits, vegetables, grains or legumes. Thus, dietary sources richest in vitamin B12 (per serving) include organ meats, followed by beef, cheese, chicken, egg, fish (clam, oyster), milk and nutritional supplements.