It is used in medicine as an injection (the common trade name is Neo-Cytamen) to treat pernicious anaemia, pins and needles sensation of the feet and some eye disorders (eg. optic neuropathy).
The recommended daily dietary intake is 0.3 µg. in infants, 1.5 µg. in children and 2.0 µg. in adults.
Side effects are usually minimal, but uncommonly it may cause low blood potassium, diarrhoea, itch, swelling sensation and a blood clot in a vein. It may interact with oral contraceptives, chloramphenicol and folic acid. An overdose is unlikely to have any adverse effects.
Several chemical variations of vitamin B12 exist including cyanocobalamin and hydroxycobalamin. They are identical in their actions and use. Cyanocobalamin was the original form of vitamin B12 used medically. It is essential for the formation of red blood cells, normal growth, and normal fat and sugar metabolism. In pernicious anaemia, the stomach stops producing intrinsic factor, which is usually used by stomach to absorb vitamin B12.
Remember, vitamins are merely chemicals that are essential for the functioning of the body, and if taken to excess, act as a drug.