Vitamin B-12 Toxicity

January 16, 2023

Vitamin B12, or cobalamin, is named so because it contains cobalt in its structure. Only bacteria, archaea, and the human microbiome can synthesise vitamin B12, while plants cannot, which is why it is difficult to obtain the same from plant sources. The main natural sources of vitamin B12 are clams, mussels, liver, mackerel, crab, salmon, beef, and chicken. Vitamin B12 is present in food products of animal origin only and not of plant origin. Vegetarians and the elderly are at higher risk of vitamin B12 deficiency. Specifically, vegans are prone to B12 deficiency due to the lack of B12 in higher plants (plants with complexity in vascular tissue). The RDA for B12 is around 2.5 mcg/day for adults, 1.8 mcg for older children, 2.6 mcg for pregnant women, and 2.8 mcg for lactating mothers, which is the lowest of all the vitamins 1

Role of Vitamin B12

Vitamin B12 is essential for a variety of body processes, including metabolism, the production of red blood cells, and the defence of the brain system. It is necessary for the production of DNA and RNA and aids in the breakdown of fats and carbs. Megaloblastic anaemia, a kind of anaemia in which red blood cells are bigger than normal, can be prevented with vitamin B12. It is crucial for preserving normal blood levels of homocysteine, an amino acid associated with an elevated risk of heart disease. Maintaining healthy neurological and cognitive processes, such as memory and focus, requires vitamin B12 2

A lack of vitamin B12 can result in a variety of symptoms, including fatigue, depression, memory issues, difficulty focusing, and trouble walking. Along with eyesight abnormalities, those who are vitamin B12 deficient may also suffer tingling or numbness in their hands and feet. Anaemia, a disease in which the body is unable to create enough healthy red blood cells, can also result from a vitamin B12 deficit and cause weakness and exhaustion. The neurological system might become permanently harmed by a vitamin B12 deficiency over time 3.

Early detection and treatment of Vitmain B12 deficiency can often cure many hematologic and neuropsychiatric problems. Macrocytic anaemia is frequently brought on by vitamin B12 deficiency, which has also been linked to various neuropsychiatric problems. The effects of B12 deficiency on hyperhomocysteinemia and the development of atherosclerosis are currently being studied. Serum vitamin B12 levels are routinely measured to diagnose vitamin B12 deficiency; however, around 50% of people with subclinical illnesses have normal B12 levels 4.

The normal B12 level ranges are 118 to 701 picomoles per litre (pmol/L) or 160 to 950 picograms per millilitre (pg/mL) 5 6

However, the toxicity of high B12 levels is hardly discussed and is uncommon. Generally, high amounts of B12 are removed from the body through urine because this type of vitamin falls into the category of a water-soluble vitamin 7.

Symptoms of high B12 levels in the blood include:

  • Liver diseases like cirrhosis or hepatitis
  • Myeloproliferative disorders (malignant diseases that are associated with increased production of blood cells)
  • Renal impairment 
  • Anxiety
  • Headache
  • Tingling sensation in hands and feet 7

Tests to diagnose a high vitamin B12 level

To confirm high vitamin B12 levels, patients are advised to take Holo TC and MMA Test 6

Holo TC: Holotranscobalamine is a B12 status marker. This test detects vitamin B12 deficiency anaemia and other types of nerve damage.

MMA test: Methylmalonic acid is a substance made in small amounts during metabolism. Vitamin B12 plays a crucial role in its metabolism. If our body doesn’t have enough vitamin B12, it will make excess MMA. Low MMA levels can signify vitamin B12 toxicity 7.

Homocysteine level test: It calculates the body's homocysteine levels, an amino acid. The examination is frequently used to identify vitamin B6, B9, or B12 deficiencies. The lower level of homocysteine indicates a high level of B12 8.

What is the treatment for high levels of vitamin B12? 

  • Decrease the consumption of food products rich in Vitamin B12 like meat, fish, dairy products etc. 
  • Regularly monitor B12 levels. If there’s B12 toxicity, discontinue B12 supplementation for a couple of weeks.
  • Injections generally increase the level of vitamin B12 more than supplements. Check B12 status after every dose. 

Vitamin B12 requirement and chances of toxicity in pregnancy:

The formation of healthy red blood cells as well as the growth and operation of your baby's brain and spinal cord depend on vitamin B12. Additionally, vitamin B12 aids in the synthesis of DNA, which makes up every cell in infants. According to certain research, kids delivered to mothers who did not get enough vitamin B12 during pregnancy may be more likely to suffer neural tube abnormalities. Women who are pregnant require a little bit more vitamin B12 than usual 9. Normally, the daily requirement for Vitamin B12 is 2.4 mcg/day for men and women in general, but for pregnant and lactating women, it is 2.6 mcg/day to 2.8 mcg/day 1

Naturally occurring vitamin B12 may be found in a wide variety of meals that come from animals. Despite the fact that there have been few published studies on the subject, there is no evidence or reason to believe that taking more vitamin B12 (in any form) during pregnancy increases the risk of congenital malformations or has any other negative effects on the foetus 10.


(1) Vitamin B12: Functions, Food Sources, Deficiencies and Toxicity. Netmeds. (accessed 2022-12-15).

(2) Romain, M.; Sviri, S.; Linton, D. M.; Stav, I.; van Heerden, P. V. The Role of Vitamin B12 in the Critically Ill--a Review. Anaesth. Intensive Care 2016, 44 (4), 447–452.

(3) Shipton, M. J.; Thachil, J. Vitamin B12 Deficiency - A 21st Century Perspective. Clin. Med. Lond. Engl. 2015, 15 (2), 145–150.

(4) Smith, A. D.; Warren, M. J.; Refsum, H. Vitamin B12. Adv. Food Nutr. Res. 2018, 83, 215–279.

(5) Signs and Symptoms of High B12 That You Should Tell Your Doctor | livestrong. (accessed 2022-12-15).

(6) Vitamin B12 Test: High Blood Levels. (accessed 2022-12-15).

(7) Methylmalonic Acid (MMA) Test: MedlinePlus Medical Test. (accessed 2022-12-15).

(8) Robertson, J.; Iemolo, F.; Stabler, S. P.; Allen, R. H.; Spence, J. D. Vitamin B12, Homocysteine and Carotid Plaque in the Era of Folic Acid Fortification of Enriched Cereal Grain Products. CMAJ Can. Med. Assoc. J. J. Assoc. Medicale Can. 2005, 172 (12), 1569–1573.

(9) Hinga, E.; M.S.; R.D.; December 6,  registered dietitian |; 2021. Vitamin B12 during pregnancy. BabyCenter. (accessed 2023-01-02).

(10) USE OF VITAMIN B12 IN PREGNANCY. (accessed 2023-01-02).

Chandra Biswas

She is specialized in Cell and Molecular biology and is an enthusiastic researcher. She believes in sharing practices which are scientifically researched and proven.

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