It is known that lack of vitamin B12 in the body can cause neurological disorders.
Due to the vitamin B12 deficiency can lead to Megaloblastic anemia. It is caused by a secondary deficit of reduced folate. Neurological disorder that accompanies vitamin B12 deficiency may be a secondary result of the relative deficit of methionine.
There is evidence that a deficiency of vitamin B12 can occur in a large portion of the population, even in the absence of visible signs of this deficiency (such as anemia). Development of new methods and more sensitive tests made possible to determine this issue.
Overdose (hypervitaminosis, intoxication)
There is not much evidence of the toxicity of this vitamin, especially if consumed by food, because very small amount of this vitamin is absorbed.
However, if the vitamin B12 is received by intravenous injections it may cause skin problems. This vitamin is eliminated from the body within 1-2 weeks.