What are the symptoms of a vitamin B12 deficiency?

What is vitamin B12?

Vitamin B12 is also known as cobalamin, extrinsic factor (EF) of Castle, and anti-pernicious anaemia factor. Vitamin B12 is a water-soluble vitamin that can be found naturally in some foods, added to others, or purchased as a dietary supplement. Compounds with vitamin B12 are referred to as "cobalamins'' since vitamin B12 includes the mineral cobalt.³ The metabolically active forms of vitamin B12 are methylcobalamin and 5-deoxyadenosylcobalamin. However, after being converted to methylcobalamin or 5-deoxyadenosylcobalamin, two additional forms, hydroxocobalamin and cyanocobalamin, become biologically active.²


Vitamin B12 is required for the formation and maintenance of nerve and red blood cells, as well as the support of the nervous system. The foundation of all cells, DNA, also requires this vitamin for formation.⁷

Vitamin B12 is necessary for the functioning of the central nervous system, myelination, and development, as well as the production of healthy red blood cells and DNA synthesis.² Methionine synthase and L-methylmalonyl-CoA mutase are the two enzymes that require vitamin B12 as a cofactor. Homocysteine is converted to the essential amino acid methionine by methionine synthase.³ S-adenosylmethionine, a universal methyl donor for about 100 distinct substrates, including DNA, RNA, proteins, and lipids, that requires methionine for its formation. In the metabolism of propionate, a short-chain fatty acid, L-methylmalonyl-CoA mutase transforms L-methylmalonyl-CoA to succinyl-CoA.¹

The average recommended amount of vitamin B12

According to the Dietary Reference Intakes (DRIs) guidelines produced by the Food and Nutrition Board (FNB) of the National Academies of Sciences, Engineering, and Medicine, the average daily need is 1-2 micrograms. This is increased to 2 mg/day during pregnancy and breastfeeding. Folic acid users should also supplement with vitamin B12. Additionally, B12 supplementation is recommended for the elderly.²

Vitamin B12 deficiency causes

There are different conditions or situations that would result in vitamin B12 deficiency, including:

1. Nutritional: Foods rich in B12 tend to be animal-based products, including beef, fish and dairy products. Vitamin B12 insufficiency is extremely widespread in India, particularly among vegetarians from lower socioeconomic groups. Curd/milk is the only source of B12 in a vegetarian diet, and lower-income people may not be able to access it.⁴

2. Reduced absorption: Gastrectomy, ideal resection, and malabsorption syndromes can diminish the surface area for absorption.⁴

3. Addisonian pernicious anaemia: It is widespread in European countries. It was pernicious (fatal) when Thomas Addison first reported it in 1849, and there was no known cure. It mainly affects those over the age of 40. It is an autoimmune disorder with a strong genetic component that causes the production of antibodies against intrinsic factors (IF).  As a result, IF becomes inadequate, causing poor B12 absorption.⁴

4. Pregnancy: Another prevalent reason for vitamin B12 insufficiency in India is increased vitamin requirements during pregnancy.⁴

5. Fish tapeworm: Diphillobothrium latum infection is frequent in Scandinavian nations, where eating live fish is considered a delicacy. This tapeworm has a strong affinity for B12, reducing the amount of vitamin available. ⁴

6. Digestive disorders: Digestive disorders such as Crohn's disease and celiac disease can hinder your body from adequately absorbing vitamin B12.⁶ Gastric atrophy can also lead to a deficiency of IF and decreased B12 absorption.⁴

7. Gastrointestinal surgery: Individuals who have had gastrointestinal surgery, such as a gastric bypass (weight loss surgery), may have trouble absorbing vitamin B12.⁶

8. Alcohol use disorder: This ailment can harm your digestive system and leave you deficient in vitamin B12.⁶

9. Transcobalamin II deficiency: This rare hereditary condition prevents vitamin B12 (commonly known as cobalamin) from being transported throughout the body.⁶

Symptoms of a vitamin B12 deficiency anaemia

A lack of vitamin B12 can result in physical, neurological, and psychological consequences. Vitamin B12 insufficiency symptoms can appear gradually and worsen over time. Despite having a low level of vitamin B12 in their systems, some people may experience no symptoms. People with vitamin B12 insufficiency without anemia (lack of red blood cells), can experience neurological symptoms and/or damage. The most common symptoms include:

  • Fatigue, or feeling excessively tired or weak
  • Pale or yellowish skin colouration due to lack of RBCs
  • Difficulty walking or speaking in a usual way
  • Headaches
  • Irregular heartbeats
  • Numbness or tingling in the hands or feet
  • Muscle weakness
  • Memory loss


Long-term B12 deficiency can put you at risk of developing a variety of health issues. One possible explanation is that the body requires vitamin B12 to metabolise homocysteine, an amino acid used to build proteins in the body. Experts have discovered associations between high homocysteine levels and diseases like dementia and cardiovascular disease.⁷

Low vitamin B12 levels can have a long-term impact on the brain function and raise the risk of cognitive decline. It may cause decreased school performance in youngsters, irritability, memory loss, depression, and dementia (depending on the person's age). Some studies have observed homocysteine levels to be greater in Alzheimer's and dementia patients. Researchers reported the findings of a 2012 study that included 121 participants aged 65 and up. They first analysed the subjects' blood for vitamin B12 levels and other B12 deficiency signs. They also tested their memory and other cognitive abilities. They did MRI brain scans after 52 months. They measured the size of the brain and looked for any signs of injury. After 52 months, those who showed the most vitamin B12 deficiency in the initial examinations were substantially more likely to have poorer cognitive test scores and smaller total brain capacity.

According to studies, 20–30% of persons with B12 deficiency suffer neurological impairment, including nerve damage, spinal cord injury, and neurological issues that affect mental health.

High amounts of homocysteine have also been discovered in persons with coronary heart disease and stroke. However, there is little research supporting the use of B12 supplements to prevent heart disease. Some people use B12 pills to increase their energy levels and athletic performance. However, supplements appear to help only if a person already has a deficiency.


A GP may typically diagnose vitamin B12 or folate deficiency anaemia based on your symptoms and the blood test results.  Vitamin B12 deficiency can be difficult to detect because symptoms can be absent or might be confused with other nutritional deficiencies. In those who are at high risk of developing vitamin B12 deficiency, healthcare providers will normally conduct routine blood tests. A complete blood count (CBC) and a vitamin B12 blood test level are used to identify vitamin B12 insufficiency. Vitamin B12 deficiency is diagnosed when the level of vitamin B12 in a person's blood is less than 150 per millilitre. ⁶

The blood tests are performed to help identify if the suspected person is affected with vitamin B12 or folate deficiency. These tests determine whether you have a lower level of haemoglobin (a substance that carries oxygen) than usual, whether the number of RBCs in your blood is larger than your vitamin B12 level, folate level and normal RBCs count. ⁵ However, some people may experience issues with their regular vitamin levels, or may exhibit low levels despite having no symptoms. This is why it's critical that all the symptoms be considered when a patient is diagnosed. The current widely used blood test only evaluates the overall quantity of vitamin B12 in your blood, which is a major disadvantage in assessing vitamin B12 levels. This means it cannot distinguish between "active" forms of vitamin B12 that your body can use and "inactive" forms that it cannot use.  A blood test may suggest that the person would have appropriate B12 levels despite the fact that your body cannot use much of it if a large quantity of vitamin B12 in your blood is inactive. Some blood tests can help establish whether the vitamin B12 in your blood can be utilised by your body, but they aren't commonly available yet.⁸

Once the patient has been diagnosed with vitamin B12 deficiency, he or she may be referred to a specialist by the GP. If you have vitamin B12 or folate deficiency anaemia and your GP is unsure of the cause, you're pregnant, or your symptoms imply your nervous system has been impacted, you should see a haematologist.  A nutritionist should be consulted if your doctor feels you are deficient in vitamin B12 or folate because your digestive system is not adequately absorbing the nutrition. Alternatively, you will be advised to see the dietitian if your doctor feels you are deficient in vitamin B12 or folate as a result of a bad diet. They will be able to help you develop a personalised meal plan to boost your vitamin B12 or folate levels.⁸


Food sources of vitamin B12

The richest food source for vitamin B12 is the liver. Alternatively, curd, which is suitable for vegetarians, is an excellent option for vitamin B12 as it contains the probiotic bacteria lactobacillus, which produces B12. On the other hand, vegetables do not contain vitamin B12.


You'll need vitamin B12 injections at first if you have pernicious anaemia or have difficulties absorbing the required quantity of vitamin B12. After that, you may need to continue getting shots and take large amounts of a supplement orally or nasally. For those who do not eat animal products, there are other options to take a B12 supplement. If you lack vitamin B12, you can supplement with vitamin B12-fortified cereals, take a B12 injectable, or take a high-dose oral B12 vitamin.

Vitamin B12 deficiency in older persons will almost certainly require a daily B12 pill or a B12-containing multivitamin.  Treatment generally cures the condition for most people. However, any nerve damage caused by the deficit may be permanent. ⁵


Vitamin B12 insufficiency can be avoided by eating a well-balanced diet that includes meat, chicken, cheese, and milk. If you don't eat animal products or have a medical condition that prevents your body from absorbing nutrients properly, you can get vitamin B12 through a multivitamin or other supplement, as well as fortified meals.  If you decide to take vitamin B12 supplements, talk to your doctor so they can determine how much you need and whether or not they will interfere with any medications you're taking.

When to see a doctor

If you are experiencing symptoms of a vitamin B12 deficiency, you should contact your doctor. Clinical symptoms and the blood test results are often used to identify these illnesses. It's critical to diagnose and treat vitamin B12 or folate deficiency anaemia as soon as possible to avoid any complications due to delays in the treatment.


Deficiency in vitamin B12 or B9 (also known as folate) anaemia develops when the body produces excessively big red blood cells that are unable to function correctly due to a deficiency of vitamin B12 or folate. Vitamin B12 and folate have critical roles in the body, including maintaining the health of the neurological system. A lack of either of these vitamins might result in a variety of issues. Although rare, vitamin B12 or folate insufficiency (with or without anaemia) might cause neurological problems, especially if an individual has been lacking these vitamins for a long time. Vitamin B12 is therefore necessary for preventing anaemia and sustaining neurological health. It could help in the prevention of dementia, heart disease, infertility, and other potential pregnancy complications and birth abnormalities. Some symptoms improve with proper treatment, but others, such as complications in the nervous system, might be permanent.

A well-balanced diet can help prevent deficiencies. People who only eat plant-based diets should think about adding fortified foods or consulting a doctor about supplements. If an individual has pernicious anaemia, Crohn's disease, or any condition that limits their capacity to absorb vitamin B12, a doctor may suggest taking supplements.


  1. Allen, Lindsay H. “Vitamin B-12.” Advances in Nutrition (Bethesda, Md.), vol. 3, no. 1, Jan. 2012, pp. 54–55. PubMed, https://doi.org/10.3945/an.111.001370.
  2. Institute of Medicine (U.S.), et al., editors. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B₆, Folate, Vitamin B₁₂, Pantothenic Acid, Biotin, and Choline. National Academy Press, 1998.
  3. Office of Dietary Supplements - Vitamin B12. https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/. Accessed 7 May 2022.
  4. Vasudevan, D. M., et al. Textbook of Biochemistry for Medical Students. Seventh edition, Jaypee Brothers Medical Publishers (P) LTD, 2013.
  5. “Vitamin B12 Deficiency: Causes, Symptoms, and Treatment.” WebMD, https://www.webmd.com/diet/vitamin-b12-deficiency-symptoms-causes. Accessed 7 May 2022.
  6. “Vitamin B12 Deficiency: Symptoms, Causes & Treatment.” Cleveland Clinic, https://my.clevelandclinic.org/health/diseases/22831-vitamin-b12-deficiency. Accessed 7 May 2022.
  7. Vitamin B12 Deficiency: Symptoms, Complications, and More. 27 Nov. 2020, https://www.medicalnewstoday.com/articles/172774.
  8. “Vitamin B12 or Folate Deficiency Anaemia.” Nhs.Uk, 20 Oct. 2017, https://www.nhs.uk/conditions/vitamin-b12-or-folate-deficiency-anaemia/.
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Sunita Baro

Master's degree - Public Health, Newcastle University, England
Sunita is passionate about serving a large community and eliminating health inequities around the globe.
Experienced as a Medical Laboratory Assistant, Healthcare Science Associate and Healthcare Assistant.

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