<unk>  deficiency can potentially cause severe and irreversible damage, especially to the brain and nervous system.
The daily cobalamin (vitamin<unk>  requirement is  1-2 mcg. An intake of 5-20 mcg/day is usual in a Western diet. The total body can store 2-5 mg. 60% of vitamin<unk>  is efficiently absorbed via the ileum when it is bound to intrinsic factor (IF).
The first deficiency symptom that was discovered was anemia characterized by enlarged blood corpuscles, so-called megaloblastic anemia.
The anemia is thought to be due to problems in DNA synthesis, specifically in the synthesis of thymine, which is dependent on products of the MTR reaction. Other cell lines such as white blood cells and platelets are often also low.  Bone marrow examination may show megaloblastic hemopoiesis.  The anemia is easy to cure with vitamin<unk> .
Far more serious is the damage to the nervous system that may occur due to deficiency.
Vitamin B12 deficiency
Vitamin A deficiency
Copper deficiency
Biotin deficiency
Zinc deficiency
Chromium deficiency
Manganese deficiency 
Folate deficiency