Some severe illness can result from a long-term deficiency of vitamin B12 including severe anemia and irreversible spinal cord degeneration with neurological symptoms. Because these symptoms don't appear until after many months of poor B12 intake, it is essential to keep an eye on intake to prevent the situation getting out of control.
People eating a vegan diet are more at risk for vitamin B12 deficiency as most food sources for B12 are animal in origin.
Unfortunately for vegetarians, there are very few non-animal sources of Vitamin B12 as plant matter contains virtually none. If you are a vegetarian, include a vitamin B12 supplement in your diet or eat non-animal foods that have been fortified with Vitamin B12.
Good sources of B-12