According to the National Institutes of Health Medline Plus, a normal vitamin B12 level ranges from 200 to 900 picograms per milliliter. Any level below 200 picograms per milliliter is considered a B12 deficiency, most often seen as a side effect of pernicious anemia.
According to Harvard Health Publications, vitamin B12 deficiency also occurs as a result of a vegetarian diet or weight-loss surgery. Vitamin B12 deficiency sometimes comes on gradually, and it can be difficult to diagnose. Symptoms can include: cognitive difficulties, difficulty walking, numbness, fatigue, weakness and paranoia, plus jaundice. A B12 deficiency can cause a swollen tongue as well.
A blood test is required to confirm vitamin B12 deficiency. Mild B12 deficiency can be corrected by taking a daily multivitamin, while more severe cases may require high-dose pills or weekly shots of B12.