Vitamin B12 deficiency is a common but under-recognized, yet easily treatable disorder in older adults. Although several causes exist, food-cobalamin malabsorption is now believed to be the most common etiology. Complications of vitamin B12 deficiency are myriad, ranging from lethargy and weight loss to dementia. Causes of deficiency include failure to separate vitamin B12 from food protein, inadequate ingestion, absorption, utilization, and storage as well as drug-food interactions leading to malabsorption and metabolic inactivation. The roles of B12 deficiency, elevated homocysteine and elevated methylmalonic acid in various disease states are still evolving. Timely screening and replacement of vitamin B12 will help prevent many complications.