What does vitamin B-12 do?

Answer 1

Gives you energy and speeds up your metabolism.
B12 is crucial to your body, as it helps to support healthy brain and nervous system functions. Without it, you could suffer brain, spine, and nervous system damage or failure, along with some blood problems (pernicious anemia).

Answer 2

Vitamin B12 is essential for the normal circulation of blood, the functioning of the brain and nervous system. It helps maintain the life of every cell in your body and keeps nerve cells healthy. Without B12 the bone marrow- a sort of jelly between your bones- cannot produce the 100 million blood cells it makes every minute. Vitamin B12 is used to treat Cyanide poisoning and vitamin B12 deficiency. It is also given as part the Shilling test which detects Pernicious anemia, a disease that destroys the red blood cells in your stomach as well as giving B12 deficiency.

Deficiency can create fatal damage to the brain and nervous system.

The recommend daily intake for an adult is 2-3 micrograms. Milligrams are bigger than micrograms so you don't have to have a lot but not enough is fatal.