Southern Cross Medical Library

Southern Cross Medical Library

Vitamin B12 is necessary for making red blood cells and is important for the health of nerve cells and the formation of genetic material. Initial symptoms of vitamin B12 deficiency include dizziness and fatigue. Without treatment, deficiency can result in anaemia (lack of healthy red blood cells), damage to the nervous system, and other health issues.

General information

Vitamin B12, also called cobalamin, is a water-soluble vitamin found in certain foods: meat, fish, milk products and eggs. Vegetables alone are an inadequate source of vitamin B12.

According to a New Zealand Adult Nutrition Survey, eight per cent of the population may be vitamin B12 deficient. The survey also revealed that inadequate vitamin B12 levels are 10 times more common among females than males.


Vitamin B12 deficiency is most commonly caused by a lack of intrinsic factor – a substance produced in the stomach that is necessary for the absorption of vitamin B12 from food. Some people are unable to produce enough intrinsic factor while others may have a health condition that destroys it. The inability to absorb vitamin B12 due to a lack of intrinsic factor is known as pernicious anaemia.

Other causes of vitamin B12 deficiency include:

  • Not eating vitamin B12-rich foods. This particularly affects vegans who do not eat animal products (meat, fish, milk, eggs, butter, cheese, and other dairy products). However, due to the liver typically storing three to five years’ worth of vitamin B12, it is difficult to become deficient from diet alone
  • Poor absorption or use of vitamin B12 because of gastric problems such as coeliac disease (gluten intolerance), inflammation of the stomach (gastritis), or pancreas problems
  • Poor absorption can also occur after stomach and intestinal surgery, and in inflammatory bowel diseases such as Crohn's disease
  • Medication that interferes with the absorption of vitamin B12 (e.g., metformin, which is used to treat diabetes, and proton-pump inhibitors and H2 receptor blockers, which are used to treat heartburn and indigestion)
  • Heavy alcohol consumption
  • Rare congenital problems (abnormal conditions present from birth).

Research studies have indicated that certain genetic (inherited) factors may influence the development of vitamin B12 deficiency in some people.


Many of the initial symptoms of vitamin B12 deficiency are associated with anaemia, where the body's cells do not get enough oxygen from the blood. Vitamin B12 deficiency anaemia, due to insufficient vitamin B12 in the diet and/or inability to absorb or process it, results in the body making faulty red blood cells that have reduced ability to carry oxygen around the body.

Symptoms of vitamin B12 deficiency anaemia include:

  • Fatigue
  • Dizziness
  • Muscle weakness
  • Paleness
  • Shortness of breath.

If left untreated, vitamin B12 deficiency can cause progressive damage to the nervous system – especially the nerves outside the brain and spinal cord. When the spinal cord is involved, the first symptoms include difficulty in feeling vibrations in the feet, loss of position sense, and loss of muscle co-ordination (ataxia). Nerve damage due to deficiency can also lead to hearing loss.

Other symptoms of untreated vitamin B12 deficiency may include:

  • Weight loss
  • Heart and blood vessel disease
  • An enlarged spleen and liver
  • Exaggerated reflexes
  • Mild depression and confusion
  • Hallucinations, personality, and mood changes
  • Dementia
  • Irritability
  • Damage to the optic nerve.


Vitamin B12 deficiency can be difficult to diagnose due to symptoms not always being present or because they are non-specific (especially fatigue and dizziness) and similar to those of other conditions, including other nutritional deficiencies.

Diagnosis of vitamin B12 deficiency may include taking a blood sample to examine the red blood cells and to measure the levels of vitamin B12. Tests to determine blood levels of methylmalonic acid and homocysteine may also be done. High blood levels of either of these substances may indicate vitamin B12 deficiency. Blood folate levels may be measured to differentiate vitamin B12 deficiency from folate deficiency (another type of nutritional deficiency).


In most cases of vitamin B12 deficiency, intramuscular injections of vitamin B12 are given. These injections bypass barriers to absorption of vitamin B12 from food. To begin with, regular injections of 1,000 micrograms are usually given every week for four to six weeks. This frequency allows the body’s physiology to start producing red blood cells normally. It also allows the reserves of vitamin B12 that the liver normally holds to increase.

The injections do not solve the underlying cause of the deficiency and, depending on the underlying cause, it may be necessary to continue having B12 injections for life. It is usually recommended that a maintenance dose of 1,000 micrograms is given every three months.

It is recommended that vegans take vitamin B12 supplements or vitamin B12-enriched nutritional yeast. Oral iron supplements can be prescribed if an iron deficiency is also present.

Side effects

Adverse effects resulting from vitamin B12 supplementation are rare. Hypersensitivity (anaphylaxis) is exceptionally rare and may include swelling, itching, and shock. Very high doses of vitamin B12 may sometimes cause acne. Other uncommon side effects include skin rash, hot flushes, nausea, dizziness, and cardiac arrhythmias.


It is important to note that vitamin B12 from supplements can be destroyed if the supplements are taken within an hour of large amounts of vitamin C (ascorbic acid). As they do with dietary sources of vitamin B12, metformin, proton-pump inhibitors, and H2-receptor blockers can interfere with the absorption of vitamin B12 from supplements. Absorption can also be reduced by deficiencies in folic acid, iron, or vitamin E. Use of nicotine or excessive alcohol can also deplete vitamin B12 levels.

Improved absorption of vitamin B12 occurs when it is taken with other B vitamins or calcium. Talk to your doctor about other possible interactions with medications you may use.


Langan, R.C., Goodbred, A.J. (2017). Vitamin B12 deficiency: Recognition and management. Am Fam Physician. 2017 Sep 15;96(6):384-389.

Mayo Clinic (2021). Vitamin B12 (Web Page). Rochester, MN: Mayo Foundation for Medical Education and Research. [Accessed: 18/11/22]

Johnson, L.E. (2022). MSD Manual: Vitamin B12 deficiency (Web Page). Rahway, NJ: Merck & Co.,-dependency,-and-toxicity/vitamin-b12-deficiency [Accessed: 16/12/22]

National Institutes of Health Office of Dietary Supplements (2018). Vitamin B12 Fact Sheet (Web age). Bethesda, MD: National Institutes of Health. [Accessed: 18/11/22]

University of Otago and Ministry of Health. (2011). A Focus on Nutrition: Key findings of the 2008/09 New Zealand Adult Nutrition Survey. Chapter 4. Nutrient intakes and dietary sources: Micronutrients. Wellington: New Zealand Ministry of Health.

O’Toole, M.T. (Ed.) (2017). Vitamin B12 test (cyanocobalamin). Mosby’s Dictionary of Medicine, Nursing & Health Professions (10th ed.). St Louis, MI: Elsevier.

Updated: November 2022




The purpose of the Southern Cross Medical Library is to provide information of a general nature to help you better understand certain medical conditions. Always seek specific medical advice for treatment appropriate to you. This information is not intended to relate specifically to insurance or healthcare services provided by Southern Cross.