Your body needs vitamin B12 to make red blood cells. In order to provide vitamin B12 to your cells:

• You must eat foods that contain vitamin B12, such as meat, poultry, shellfish, eggs, fortified breakfast cereals, and dairy products.
• Your body must absorb enough vitamin B12. A special protein, called intrinsic factor, helps your body do this efficiently. This protein is released by cells in the stomach.

Certain health conditions can make it difficult for your body to absorb enough vitamin B12. They include:

• Pernicious anemia, a condition that occurs when your body destroys cells in your stomach that make intrinsic factor, which may lead to vitamin B12 deficiency and anemia
• Surgery that removes certain parts of your stomach or small intestine, such as some weight-loss surgeries
• Crohn disease, celiac disease, infection with the fish tapeworm, or other problems that make it difficult for your body to digest foods
  
• Alcohol use
• Taking antacids and other heartburn medicines for a long period of time
• Abuse of laughing gas (nitrous oxide)
• Long term use of the medicine metformin

A lack of vitamin B12 may be due to dietary factors, including:

• Eating a strict vegetarian diet
• Poor diet in infants
• Poor nutrition during pregnancy

You may not have symptoms. Symptoms may be mild.

Symptoms may include:

• Diarrhea or constipation
• Fatigue, lack of energy, or lightheadedness when standing up or with exertion
• Loss of appetite
• Pale skin
• Feeling irritable
• Shortness of breath, mostly during exercise
• Swollen, red tongue or bleeding gums

If you have low vitamin B12 level for a long time, it can cause nerve damage. Symptoms of nerve damage include:

• Numbness and tingling of hands and feet
• Loss of balance
• Problems concentrating
• Confusion or change in mental status (dementia) in severe cases
• Psychosis (losing contact with reality)
• Hallucinations

The health care provider will perform a physical exam. This may reveal problems with your reflexes or sensations.

Tests that may be done include:

• Complete blood count (CBC)
• Reticulocyte count
• Lactate dehydrogenase (LDH) level
• Serum bilirubin level
• Serum vitamin B12 level
• Serum methylmalonic acid (MMA) level
• Serum homocysteine level (amino acid found in blood)
• Anti-intrinsic factor antibody
• Anti-parietal cell antibody

Other procedures that may be done include:

• Esophagogastroduodenoscopy (EGD) to examine the stomach and small intestine
• Bone marrow biopsy if the diagnosis is not clear

Treatment depends on the cause of B12 deficiency anemia. The goal of treatment is to increase your vitamin B12 level.

For people who have a low level of B12 due to a restricted vegetarian diet, taking B12 pills may resolve the issue.

For people with a health condition that causes B12 deficiency, treatment may include a shot of vitamin B12, often once a month. If you have a very low level of B12, you may need more shots in the beginning. It is possible you may need shots regularly for the rest of your life.

Your provider will also recommend that you eat a variety of foods.

People with this type of anemia often do well with treatment.

Long-term vitamin B12 deficiency can cause nerve damage. This may be permanent if you do not start treatment promptly after your symptoms begin. If nerve damage is severe or long-lasting, some damage may be permanent.

Vitamin B12 deficiency anemia most often responds well to treatment. It will likely get better when the underlying cause of the deficiency is treated.

A woman with a low B12 level may have a false positive Pap smear. This is because vitamin B12 deficiency affects the way certain cells (epithelial cells) in the cervix look.

Contact your provider if you have any of the symptoms of anemia.

Eating a well-balanced diet can help ensure you get enough vitamin B12 from the foods you eat. However, it can't prevent anemia in people with certain health conditions that prevent their body from using vitamin B12 properly.

Shots of vitamin B12 can prevent anemia if you've had a surgery known to cause vitamin B12 deficiency.

Early diagnosis and prompt treatment can reduce or prevent complications related to a low vitamin B12 level.

There is no evidence that taking B12 supplements will prevent or treat dementia, Alzheimer disease, or other health conditions such as heart disease.

Megaloblastic macrocytic anemia; Cobalamin deficiency anemia

Antony AC. Megaloblastic anemias. In: Hoffman R, Benz EJ, Silberstein LE, et al, eds. Hematology: Basic Principles and Practice. 8th ed. Philadelphia, PA: Elsevier; 2023:chap 40.

Means Jr. RT. Approach to the anemias. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 144.

National Institutes of Health website. Vitamin B12: fact sheet for health professionals. ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/. Updated March 26, 2024. Accessed August 17, 2024.

Stabler SP. Megaloblastic anemias. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 150.

Updated by: Todd Gersten, MD, Hematology/Oncology, Florida Cancer Specialists & Research Institute, Wellington, FL. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team. Editorial update 08/19/2024.