A condition in which the body has too little vitamin B-12, a nutrient that is needed to make red blood and nerve cells and DNA.

Over time, a vitamin B-12 deficiency might lead to nerve damage, mood disorders and dementia if it's not treated. The deficiency has various causes. These include trouble absorbing vitamin B-12 from food, not eating animal-based foods and long-term use of certain medicines.

Symptoms of a vitamin B-12 deficiency can include a swollen tongue, fatigue, weight loss, weakness, dizziness, shortness of breath, and a racing, fluttering or pounding heartbeat. These symptoms can happen when the deficiency causes a low level of red blood cells. This condition is called anemia.

Treatment for a vitamin B-12 deficiency often involves taking the vitamin in supplement form. Some people receive injections of vitamin B-12 in a muscle. Others take high doses of the vitamin by mouth.