• Diarrhea or constipation • Fatigue, lack of energy, or light-headedness when standing up or with exertion • Loss of appetite • Pale skin • Problems concentrating • Shortness of breath, mostly during exercise • Swollen, red tongue or bleeding gums If you have low vitamin B12 levels for a long time, you can have nerve damage. Symptoms of nerve damage include: • Confusion or change in mental status (dementia) in severe cases • Depression • Loss of balance • Numbness and tingling of hands and feet