This type of deficiency causes anemia in the body, which means the body doesn't have enough red blood cells. Certain conditions include Crohn's disease, celiac disease, pernicious anemia, alcoholism and make it hard for the body to absorb enough of the vitamin. Lack of this essential vitamin can cause fatigue, and even in some severe cases paralysis and nerve damage.  Treatment with a balanced diet or B12 supplements or shots brings levels back to normal.