A vitamin B12 deficiency can lead to nerve and brain damage and anemia. Early symptoms include weakness, constipation, and depression. It can result from dietary factors, medication use, and some underlying conditions.

Vitamin B12 is vital for creating and maintaining nerve and red blood cells, it supports the nervous system, and it helps create DNA, the basis of all cells.

Without treatment, vitamin B12 deficiency can lead to anemia. It can also result in nerve and brain damage. According to the Office of Dietary Supplements (ODS), the damage may be irreversible in the long term.

This article looks at how to recognize B12 deficiency, who is at risk, and how to treat or prevent it.

Learn more about the B vitamins here.

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According to the National Health Service (NHS) in the United Kingdom, a person with vitamin B12 deficiency may develop anemia and other symptoms.

Low levels of B12 can lead to:

In infants, signs of a deficiency include:

  • not growing or developing at the expected rate
  • problems with movement
  • megaloblastic anemia

Anemia and neurological problems can lead to further symptoms and complications.


Body cells need vitamin B12 to reproduce. If a person does not have enough vitamin B12, their body cannot make enough red blood cells. This can result in anemia.

The hallmark symptom of B12 deficiency is megaloblastic anemia, in which the red blood cells are immature and larger than usual. This affects their ability to deliver oxygen effectively to the body.

Common symptoms of anemia are:

Neurological symptoms

A vitamin B12 deficiency can also lead to neurological symptoms, such as:

Long-term B12 deficiency may increase the risk of various health problems. One reason for this may be that the body needs vitamin B12 to process homocysteine.

Experts have found links between high levels of homocysteine and some health conditions, such as dementia and cardiovascular disease.

Dementia and thinking

Low levels of vitamin B12 may affect brain health in the long term and increase the risk of cognitive decline.

Depending on the person’s age, it may lead to:

Some research has found higher levels of homocysteine in people with Alzheimer’s and dementia.

In 2012, researchers published the results of a study involving 121 people aged 65 years and over.

First, they tested the participants’ blood to identify levels of vitamin B12 and other markers of B12 deficiency. They also assessed their memories and other thinking skills.

After 52 months, they carried out MRI scans of the participants’ brains. They checked for brain size and any signs of brain damage.

Results showed that those with the strongest signs of vitamin B12 deficiency in the initial tests were much more likely to have lower cognitive test scores and smaller total brain volumes after 52 months.

Neurological damage

Studies have shown that 20–30% of people with B12 deficiency have neurological damage, such as:

  • nerve damage
  • damage to the spinal cord
  • neurological problems that affect mental health

Cardiovascular disease

Experts have also found high levels of homocysteine in people with coronary heart disease and stroke.

However, there is not enough evidence to recommend B12 supplementation to prevent heart disease.

Energy levels

Some people use B12 supplements to boost energy levels and athletic performance. However, supplements only appear to help if a person already has a deficiency.

The amount of vitamin B12 an individual needs per day will depend on their age. People also need more B12 during pregnancy and while breastfeeding.

According to the ODS, the average daily recommended amounts for different ages in micrograms (mcg) are:

0–6 months0.4 mcg
7–12 months0.5 mcg
1–3 years0.9 mcg
4–8 years1.2 mcg
9–13 years1.8 mcg
14 years and over2.4 mcg
During pregnancy2.6 mcg
While breastfeeding2.8 mcg

A B12 deficiency can occur if a person does not consume enough of the vitamin in their diet or if their body cannot absorb it effectively during digestion.

When a person consumes food that contains vitamin B12, the body takes two steps to absorb it.

First, hydrochloric acid in the stomach separates it from the protein it binds to in food.

Then, vitamin B12 combines with another protein that the stomach produces, called intrinsic factor. Then, the intestines can absorb it.

Here are some possible causes of vitamin B12 deficiency.

1) Pernicious anemia

Pernicious anemia is an autoimmune disease that affects the gut’s ability to digest vitamin B12.

When a person has pernicious anemia, their immune system creates antibodies that attack the stomach’s lining. There, they damage cells that produce intrinsic factor.

If the stomach cannot produce intrinsic factor, the intestines will be unable to absorb vitamin B12.

2) Bowel problems

Some people may have problems absorbing vitamin B12 into their bloodstream because of a condition that affects their stomach or small intestine.

Possible causes include:

These people can experience malnutrition because they cannot absorb enough vitamins, water, and other nutrients.

3) Diet

People who follow a vegan or vegetarian diet may have a higher risk of B12 deficiency. During pregnancy, this may increase the risk of neurological damage in a fetus.

While some plant foods contain vitamin B12, it is often in a form that the body cannot absorb efficiently, according to research published in 2013.

People who follow a plant-based diet can supplement their B12 intake by eating fortified foods, such as nutritional yeast products.

4) Medications

Some medications can affect the body’s ability to digest vitamin B12, leading to a deficiency.

Examples include proton pump inhibitors (PPIs) and Histamine H2 receptor agonists (H2 blockers), which doctors prescribe to treat indigestion, heartburn, and gastroesophageal reflux disease (GERD).

PPIs prevent the stomach from producing acid, but the body needs acid to absorb vitamin B12.

5) Functional vitamin B12 deficiency

Some people appear to have enough vitamin B12 in their blood, but they do not have the proteins they need to transport it between cells. This means the body cannot use it effectively.

They can develop neurological symptoms related to B12 deficiency.

To assess for a vitamin B12 deficiency, a doctor will likely:

  • ask about symptoms
  • carry out a physical examination
  • do a blood test to measure B12 levels and check for anemia
  • do other tests to rule out other conditions and check for pernicious anemia, if necessary

Some people will have a B12 deficiency but no symptoms. A doctor may recommend regular testing for people with long-term gastrointestinal problems to ensure a deficiency does not develop.

Getting an early diagnosis can help prevent long-term complications.

Treatment will depend largely on the cause of a deficiency.

Options include increasing vitamin B12 intake through:

  • intramuscular injections
  • oral medicine
  • adding foods that are rich in B12 to the diet

Some people may need regular injections for the rest of their life.

Most people can prevent B12 deficiency through dietary choices, but this is not always the case.

People following a plant-based diet

People whose diet is largely or wholly plant-based should ensure their B12 intake is adequate.

Dietary options include:

  • fortified breakfast cereals
  • fortified nutritional yeast
  • fortified soy milk

Learn more about supplements that are suitable for people on a vegan diet.

People who eat meat or fish

For those who eat meat or fish, a balanced diet containing fish, meat, and dairy foods should have enough B12 for human requirements.

Other conditions and medical treatments

Anyone who has a chronic gastrointestinal condition or believes they may have symptoms of B12 deficiency should seek medical advice.

The doctor will seek to identify the cause and treat it accordingly.

Good sources of vitamin B12 include the following:

  • clams
  • beef liver
  • fish, such as trout, salmon, and tuna
  • nutritional yeast
  • dairy products
  • eggs
  • fortified yeasts and other foods

Get some more tips here on vitamin B12 sources.

Vitamin B12 is essential for preventing anemia and maintaining neurological health. It may help prevent dementia and heart disease.

A balanced diet can often help prevent a deficiency. People who eat only plant-based foods may consider including fortified foods or asking a doctor about supplements.

A doctor may also recommend supplements if a person has pernicious anemia, Crohn’s disease, or another condition that affects their ability to absorb vitamin B12.