Vitamin B12, a water-soluble vitamin is stored and reused more efficiently that it does other water-soluble vitamins. A deficiency is therefore caused more by poor absorption rather than by a low intake alone.
A deficiency can cause pernicious anemia that is best treated by B12 injections instead of megadoses of vitamin B12 supplements or dietary means. Deficiencies are rare but there can be marginal vitamin B12 status, particularly in older adults and/or vegetarians who consume no animal products.
A deficiency also interferes with the maintenance of myelin that coats the nerves, spinal cord, and brain. When the myelin degenerates, neurological symptoms of numbness and tingling, abnormalities in walking, memory loss, and disorientation occur. If not treated, this eventually causes paralysis and death.
FYI: Please do not take vitamin B12 supplements without getting your vitamin B12 levels checked. No toxic effects have been reported with excess intakes up to 100 ug/day but there is insufficient data to establish an Upper Tolerable Intake. The RDA for adults of all ages is 2.4 ug/day.