The human body stores several years' worth of vitamin B12, so nutritional deficiency of this vitamin is extremely rare.

However, deficiency can result from being unable to use vitamin B12. Inability to absorb vitamin B12 from the intestinal tract can be caused by a disease known as pernicious anemia. Additionally, strict vegetarians or vegans who are not taking in proper amounts of B12 by way of supplements are also prone to a deficiency state.

Low levels of B12 can cause anemia as well as numbness or tingling in the extremities and other neurologic symptoms such as weakness and loss of balance.