Vitamin B12 is an essential vitamin that is not found in plant foods. Vitamin B12 is required for many biochemical processes in the body and is required for the normal production of energy, the normal function of the immune system and for the normal function of red blood cells. It is also needed for the function of the nerves and for normal levels of homocysteine, a heart disease risk factor. Vitamin B12 is needed for normal psychological function and for cell replication.