Vitamin B12 is needed for blood formation and for cell division. Neither plants nor animals make vitamin B12. Bacteria are responsible for producing vitamin B12.
Animals get their vitamin B12 from eating foods contaminated with vitamin B12 and then the animal becomes a source of vitamin B12. Plant foods do not contain vitamin B12 except when they are contaminated by microorganisms or have vitamin B12 added to them. Therefore, vegans need to look to fortified foods or supplements to get vitamin B12 in their diet.
Although recommendations for vitamin B12 are very small, a vitamin B12 deficiency is a very serious problem leading ultimately to anaemia and irreversible nerve damage. Vegans will include sources of vitamin B12 in their diets.
Tempeh, miso, sea vegetables, and other plant foods are sometimes reported to contain vitamin B12. These products, however, are not reliable sources of the vitamin B12. Vegans should use a vitamin B12 supplement, either as a single supplement or in a multivitamin should use supplements regularly.