Symptoms. Symptoms of a deficiency in vitamin B12 can range from mild to severe. A common symptom is anemia, meaning the body is not producing enough red blood cells. Symptoms can include feeling weak and tired, lightheadedness, shortness of breath, heart palpitations and pale skin. A mild deficiency can cause tongue inflammation, making it look red and swollen or smooth. The nervous system can be impacted leading to numbness, tingling, weakness, problems with moving and walking, and even memory problems. Digestion can also be impacted with symptoms ranging from a loss of appetite to constipation and diarrhea.