Vitamin B12 (cobalamin) is important in the formation of all cells in the body, especially red blood cells and the covering of nerve cells (myelin). The body needs myelin for nerves to function properly.

Vitamin B12 is found in animal products such as meat, shellfish, milk, cheese, and eggs. Most people who eat meat are not likely to develop a vitamin B12 deficiency. There is normally enough vitamin B12 stored in a person's liver to last a year or more, even if the person does not eat any foods that contain the vitamin during that time.

Some people have a disease that makes their bodies unable to absorb vitamin B12. These people need either to get an injection of B12 once a month, or take high-dose B12 pills.

Strict vegetarians (vegans) who do not eat meat, milk, cheese, or eggs are at risk for vitamin B12 deficiency. They need a vitamin supplement containing vitamin B12.