What are the side effects of a vitamin B12 deficiency?

Vitamin B12 deficiency is defined by low levels of stored B12 in the body that can result in anemia, a lower-than-normal number of red blood cells. Symptoms tend to develop slowly and may not be recognized immediately. As the condition worsens, common symptoms of anemia include weakness and fatigue, light-headedness, palpitations and shortness of breath. If low levels of B12 remain for a long time, the condition also can lead to irreversible damage to nerve cells, which can cause the numbness and tingling in the hands and feet, imbalance, and memory loss.