Vitamin B12 Deficiency: Symptoms, Causes and Treatment

Vitamin B12 deficiency happens when the body does not have enough vitamin B12 to maintain healthy blood cells, nerves and other essential functions. It can cause tiredness, weakness, pins and needles, a sore tongue, balance problems, memory changes and anaemia. Some people develop significant nerve symptoms even when their haemoglobin and red blood cell size remain normal.

The deficiency may be caused by an autoimmune condition affecting absorption, surgery involving the stomach or bowel, digestive disorders, medicines, recreational nitrous oxide use or a diet that provides too little vitamin B12.

Treatment is usually effective, but identifying the cause matters. Someone whose diet is low in vitamin B12 may need tablets and dietary changes, while a person who cannot absorb the vitamin properly may require injections for life.

What is vitamin B12 and why does the body need it?

Vitamin B12, also called cobalamin, is a water-soluble vitamin used throughout the body. It contributes to:

• the production of healthy red blood cells;
• normal function of the brain and nervous system;
• the protective covering around nerves;
• DNA production and cell division;
• the processing of certain fatty acids and amino acids.

The body cannot make vitamin B12 itself. It must obtain the vitamin from food, fortified products or supplements.

Unlike many water-soluble vitamins, vitamin B12 can be stored in the liver in substantial amounts. This means a deficiency may take years to develop after absorption declines or dietary intake becomes inadequate.

Where does vitamin B12 come from?

Natural dietary sources are mainly foods of animal origin, including:

• meat;
• fish and shellfish;
• eggs;
• milk, cheese and other dairy products;
• liver and some other organ meats.

Plant foods do not naturally provide a reliable source of active vitamin B12. People following vegan diets therefore need fortified foods, supplements or both.

Fortified sources may include some:

• breakfast cereals;
• plant-based milks;
• nutritional yeast products;
• yeast extract spreads;
• meat-alternative products.

The amount varies considerably between brands. A product being labelled vegan does not mean it contains vitamin B12, so the nutrition label should be checked.

How is vitamin B12 absorbed?

Absorption is a multistage process. Stomach acid and digestive enzymes first release vitamin B12 from food. The vitamin then binds to a protein called intrinsic factor, which is made by specialised cells in the stomach.

The vitamin B12–intrinsic factor complex travels to the final part of the small bowel, called the terminal ileum, where it is absorbed.

A problem involving stomach acid, intrinsic factor, the stomach lining, the pancreas or the terminal ileum can therefore interfere with absorption.

What are the symptoms of vitamin B12 deficiency?

Symptoms often develop gradually and can be easy to dismiss. They overlap with many other conditions, including iron deficiency, thyroid disease, depression, sleep disorders and post-viral illness.

Possible symptoms include:

• extreme tiredness or lack of energy;
• weakness;
• breathlessness;
• headaches;
• palpitations;
• dizziness or feeling faint;
• pale skin;
• reduced appetite;
• unintentional weight loss;
• a sore, red or smooth tongue;
• mouth ulcers;
• indigestion or digestive discomfort.

These symptoms may occur when deficiency has caused anaemia, but some can also occur before anaemia develops.

Neurological symptoms

Vitamin B12 is essential for maintaining healthy nerves. Deficiency may cause:

• pins and needles;
• numbness in the hands or feet;
• burning or altered skin sensations;
• reduced sensitivity to touch, pain or temperature;
• muscle weakness;
• poor balance;
• an unsteady or unusually wide-based walk;
• loss of coordination;
• tremor;
• visual problems.

Symptoms may begin in the feet and gradually move upwards. They can affect both sides, although the pattern is not always symmetrical.

Our guide to numbness and tingling explains other possible causes of these sensations.

Memory, concentration and mood changes

Vitamin B12 deficiency may be associated with:

• difficulty concentrating;
• memory problems;
• slower thinking or “brain fog”;
• irritability;
• depression or low mood;
• anxiety-like symptoms;
• confusion;
• more severe cognitive changes in advanced cases.

These symptoms are not specific enough to diagnose deficiency. Mental health and cognitive changes have many possible causes, so assessment should consider the complete clinical picture.

Tongue and mouth symptoms

A vitamin B12 deficiency can cause glossitis, in which the tongue becomes red, sore, swollen or unusually smooth. Mouth ulcers and a burning feeling in the mouth may also occur.

Oral symptoms can have other causes, including iron or folate deficiency, infections, medicines and local dental problems.

Can B12 deficiency cause symptoms without anaemia?

Yes. This is an important point. Neurological, psychological and oral symptoms can develop even when haemoglobin is normal and there is no obvious anaemia.

Vitamin B12 deficiency should not be ruled out solely because a full blood count is normal or the red blood cells are not enlarged.

What causes vitamin B12 deficiency?

Vitamin B12 deficiency develops when intake is too low, absorption is impaired, the vitamin is inactivated or the body’s requirements are increased.

Autoimmune gastritis and pernicious anaemia

An important cause in the UK is autoimmune gastritis. In this condition, the immune system attacks cells in the stomach lining that help produce stomach acid and intrinsic factor.

Without enough intrinsic factor, the body cannot absorb vitamin B12 normally from food.

The term pernicious anaemia has traditionally been used for vitamin B12 deficiency caused by this autoimmune process. Current NICE guidance often uses the term autoimmune gastritis because deficiency can exist before anaemia develops.

Autoimmune gastritis is more likely in people who have another autoimmune condition, such as:

• autoimmune thyroid disease;
• type 1 diabetes;
• Addison’s disease;
• vitiligo.

It can also run in families. Treatment is usually lifelong because the absorption problem does not resolve through dietary changes alone.

Dietary deficiency

People following a vegan diet are at particular risk if they do not regularly consume fortified foods or take an effective supplement.

Dietary deficiency can also occur in:

• people with a very restricted diet;
• older adults with poor appetite;
• people with eating disorders;
• those experiencing food insecurity;
• people avoiding multiple food groups;
• some vegetarians who consume little dairy or egg.

However, diet should not automatically be blamed simply because someone is vegan or vegetarian. A person may have an absorption problem as well as—or instead of—low dietary intake.

Our guide to blood tests vegans and vegetarians may wish to consider covers B12 alongside iron, vitamin D and other nutrients.

Stomach surgery and weight-loss surgery

Operations that remove or bypass part of the stomach can reduce stomach acid and intrinsic factor production.

Risk can increase after:

• total or partial gastrectomy;
• gastric bypass surgery;
• some other bariatric procedures.

People who have undergone weight-loss surgery usually require long-term nutritional monitoring and may need regular vitamin B12 replacement.

Conditions affecting the small bowel

Vitamin B12 is absorbed in the terminal ileum. Absorption may therefore be reduced by:

• Crohn’s disease affecting the terminal ileum;
• surgery removing part or all of the terminal ileum;
• coeliac disease;
• some bacterial or parasitic conditions;
• other causes of malabsorption.

Coeliac disease can sometimes be overlooked when digestive symptoms are mild or absent. Our guide to coeliac disease symptoms, testing and treatment explains how it is investigated.

Age-related absorption problems

Stomach acid production may decline with age. This can make it more difficult to release vitamin B12 from food, even when the diet contains an adequate amount.

Older adults are also more likely to have autoimmune gastritis, take medicines affecting absorption or eat a limited diet.

Medicines

Some medicines can lower vitamin B12 levels, particularly when taken for a long time. These include:

• metformin;
• proton pump inhibitors such as omeprazole and lansoprazole;
• H2-receptor antagonists used to reduce stomach acid;
• some medicines used for epilepsy, nerve pain or other specialist conditions.

Do not stop a prescribed medicine because of a low B12 result. The clinician should review whether it remains necessary, whether the dose is appropriate and whether B12 replacement is needed.

Recreational nitrous oxide use

Nitrous oxide can inactivate vitamin B12 inside the body. This means serious neurological symptoms can occur even when the measured blood B12 level is not obviously low.

Frequent or heavy recreational use may cause:

• numbness and pins and needles;
• weakness;
• difficulty walking;
• poor balance;
• bladder or bowel symptoms;
• spinal cord and peripheral nerve damage.

Someone with neurological symptoms after nitrous oxide use needs prompt medical assessment. Treatment should include stopping nitrous oxide rather than simply taking supplements while continuing to use it.

Who is most at risk?

Testing may be considered when someone has symptoms or signs of deficiency together with a relevant risk factor. Clinical judgement remains important because some people have no obvious risk factor.

Risk is higher in people who:

• follow a vegan diet without reliable supplementation;
• eat a very restricted diet;
• have autoimmune gastritis or pernicious anaemia;
• have had stomach or terminal ileum surgery;
• have undergone bariatric surgery;
• have coeliac disease or Crohn’s disease;
• take metformin or long-term acid-suppressing medicines;
• use nitrous oxide recreationally;
• are older and have poor appetite or malabsorption;
• have another autoimmune condition;
• are pregnant or breastfeeding and have low dietary intake or another risk factor.

Babies and children

Babies can become deficient when the birth parent has untreated vitamin B12 deficiency, particularly during breastfeeding.

Possible signs in infants may include:

• poor feeding;
• slow weight gain;
• developmental delay or loss of skills;
• unusual sleepiness;
• reduced muscle tone;
• irritability.

Suspected deficiency in a baby or child needs prompt paediatric assessment because the developing nervous system is vulnerable.

Pregnancy and breastfeeding

Vitamin B12 is important during pregnancy for blood formation and nervous-system development.

People following vegan diets, those with malabsorption and those with previous bariatric surgery may need additional assessment and supplementation.

Do not rely solely on a general pregnancy multivitamin without checking how much B12 it contains and whether it is suitable for the identified cause of deficiency.

How is vitamin B12 deficiency diagnosed?

Diagnosis is based on symptoms, risk factors, blood results and the suspected cause. There is no single test that is perfect in every situation.

Total vitamin B12

A total serum vitamin B12 test measures the vitamin circulating in the blood. Results are generally interpreted as:

• low and consistent with deficiency;
• indeterminate or borderline;
• unlikely to indicate deficiency.

Exact reference ranges vary between laboratories and the type of test used. A result should therefore be interpreted using the range printed on the report rather than a universal internet threshold.

Active vitamin B12

An active B12 test, also called holotranscobalamin, measures the portion available to enter cells.

Some laboratories use this as the initial test. Like total B12, it has limitations and may produce an indeterminate result requiring further investigation.

Full blood count

A full blood count may show:

• low haemoglobin;
• enlarged red blood cells, described as macrocytosis;
• a raised mean cell volume, or MCV;
• low white blood cells or platelets in more severe cases.

However, a normal haemoglobin or MCV does not exclude vitamin B12 deficiency. Iron deficiency can also mask the expected increase in red blood cell size.

Read our guide to full blood count results for an explanation of haemoglobin, MCV and the other main markers.

Methylmalonic acid and homocysteine

If the initial B12 result is uncertain, additional tests may be considered.

Methylmalonic acid, or MMA, tends to increase when cells do not have enough active vitamin B12. Kidney impairment can also raise it, so the result needs context.

Homocysteine may rise in both vitamin B12 and folate deficiency and can also be influenced by kidney function and other factors.

In suspected deficiency caused by nitrous oxide, MMA or homocysteine can be particularly useful because the gas may inactivate B12 without producing a clearly low total B12 result.

Tests for autoimmune gastritis

An anti-intrinsic factor antibody test may be requested when autoimmune gastritis is suspected.

A positive result supports the diagnosis. A negative result does not completely rule it out because not everyone with autoimmune gastritis has detectable antibodies.

Further assessment may include:

• parietal cell antibody testing;
• gastrin levels;
• specialist gastroenterology review;
• gastroscopy in selected cases.

Testing for the underlying cause

Depending on the history, further tests may include:

• coeliac blood tests;
• folate and iron studies;
• thyroid tests;
• kidney and liver function;
• review of medicines;
• assessment of diet;
• investigation for gastrointestinal disease or bleeding.

Our detailed guide to B12 and folate blood-test results explains borderline values and related markers.

When should treatment begin?

In uncomplicated cases, clinicians usually take diagnostic blood samples before treatment. Supplements and injections can raise the measured B12 level and make the original diagnosis harder to confirm.

However, treatment should not be delayed when serious neurological or blood-related complications are suspected.

Examples may include:

• difficulty walking or significant loss of balance;
• progressive weakness or numbness;
• spinal cord symptoms;
• significant visual changes;
• severe megaloblastic anaemia;
• suspected deficiency following nitrous oxide use;
• a condition where delay could cause rapid deterioration.

Why early treatment matters

Blood abnormalities usually respond well to replacement. Neurological recovery is less predictable.

Nerve symptoms may take weeks or months to improve. When nerve damage has been present for a long time, recovery may be incomplete.

Persistent symptoms after starting treatment do not necessarily mean treatment has failed immediately. Nerves heal much more slowly than blood cells.

Should folic acid be taken before B12 is checked?

Folic acid can improve the anaemia caused by vitamin B12 deficiency while allowing neurological damage to continue.

For this reason, vitamin B12 status should generally be checked and B12 deficiency treated before or alongside folic acid when both deficiencies are possible.

How is vitamin B12 deficiency treated?

Treatment depends mainly on the cause, symptom severity, neurological involvement and whether the digestive system can absorb enough vitamin B12.

Replacement is given through:

• oral vitamin B12 tablets;
• intramuscular vitamin B12 injections;
• occasionally other formulations under specialist care.

Vitamin B12 injections

Hydroxocobalamin is the vitamin B12 injection commonly used in the UK. Initial injections may be given more frequently to replenish body stores.

The exact schedule depends on whether neurological symptoms are present and on current prescribing guidance.

After the initial course, maintenance injections may be given at regular intervals. Lifelong intramuscular treatment is generally needed when deficiency results from:

• autoimmune gastritis;
• total removal of the stomach;
• complete removal of the terminal ileum;
• another irreversible absorption problem.

Oral vitamin B12

High-dose oral replacement may be appropriate when:

• diet is the likely cause;
• a medicine has contributed;
• the cause is uncertain and significant malabsorption is not suspected;
• the clinician judges that enough can be absorbed;
• the person prefers tablets and can take them consistently.

NICE advises that when oral treatment is used for suspected or confirmed malabsorption, the prescribed dose should generally be at least 1 mg daily.

Over-the-counter supplements vary greatly. Some contain only a small maintenance dose, which may be insufficient to treat established deficiency.

Dietary treatment

When low intake is the cause, treatment may combine oral B12 with regular consumption of fortified foods or animal-derived sources.

A vegan diet can provide excellent overall nutrition, but B12 must be deliberately supplied through fortification or supplementation.

Improving diet alone may not correct deficiency quickly enough once symptoms or low stores are present. Replacement should follow the clinical plan.

Medicine-induced deficiency

People whose deficiency is associated with metformin, acid-suppressing medication or another treatment may be offered tablets or injections while continuing the medicine.

The prescribing clinician should consider whether the medicine remains necessary or could be changed. Treatment should not be stopped without advice.

Nitrous oxide-related deficiency

Treatment involves vitamin B12 replacement and stopping recreational nitrous oxide use.

Continuing to use nitrous oxide can inactivate replacement B12 and allow neurological injury to progress.

People with significant weakness, walking difficulty or bladder symptoms may require hospital assessment, neurological investigation and rehabilitation.

How quickly do symptoms improve?

The response varies according to symptom type, severity, cause and how long the deficiency has been present.

Blood and energy symptoms

Bone marrow production of healthy red blood cells may begin improving within days. Haemoglobin and energy levels usually take longer to recover.

Someone with severe anaemia may continue to feel tired or breathless for several weeks even when treatment is working.

Nerve symptoms

Pins and needles, numbness, balance problems and weakness may take several weeks or months to improve.

Recovery may initially feel uneven. Some symptoms improve before others, and abnormal sensations may temporarily become more noticeable as nerves recover.

Symptoms that continue to worsen after treatment should be reviewed rather than assumed to be part of normal recovery.

Memory and concentration

Cognitive symptoms may improve gradually, but it is important to consider other possible causes when they persist.

What if symptoms do not improve?

A clinician may review:

• whether the diagnosis is correct;
• whether the treatment dose and route are appropriate;
• whether injections or tablets are being received consistently;
• whether folate, iron or another deficiency is also present;
• whether another neurological or medical condition explains the symptoms;
• whether nitrous oxide use is continuing;
• whether an underlying absorption problem has been identified.

NICE advises an initial follow-up appointment, often around three months after starting treatment, or earlier when symptoms are severe. Pregnancy may require earlier review.

Should B12 blood levels be repeatedly checked after injections?

Routine repeat B12 levels are often not useful during intramuscular treatment because injections raise the blood concentration regardless of whether symptoms have fully recovered.

Follow-up generally focuses on clinical improvement, blood-count recovery where relevant and whether the treatment schedule remains appropriate.

Can vitamin B12 deficiency cause permanent damage?

Untreated deficiency can cause serious blood and neurological complications.

Possible complications include:

• peripheral neuropathy;
• difficulty walking or loss of coordination;
• spinal cord damage;
• visual impairment;
• memory and cognitive changes;
• severe anaemia;
• heart strain related to significant anaemia;
• developmental problems in infants.

Many symptoms improve with treatment, particularly when deficiency is recognised early. Long-standing neurological damage can be only partly reversible.

Autoimmune gastritis and stomach-cancer risk

People with autoimmune gastritis have a higher risk of developing certain stomach tumours than the general population. This does not mean most people will develop cancer.

New digestive symptoms should be discussed with a clinician, particularly:

• difficulty swallowing;
• persistent indigestion;
• ongoing upper abdominal pain;
• repeated vomiting;
• unintentional weight loss;
• iron-deficiency anaemia.

Monitoring is based on individual symptoms and specialist assessment rather than automatically arranging regular gastroscopy for every patient.

Pregnancy complications

Untreated deficiency during pregnancy has been associated with risks affecting both the pregnant person and the developing baby.

Anyone who is pregnant or planning pregnancy and follows a vegan diet, has malabsorption or has previously had bariatric surgery should discuss B12 intake and testing with a clinician.

When should you contact a doctor?

Arrange a GP appointment if you have persistent symptoms of deficiency, particularly when you also have a risk factor.

Request prompt assessment for:

• new or worsening pins and needles;
• numbness in the feet or hands;
• difficulty walking;
• loss of balance or coordination;
• unexplained muscle weakness;
• visual changes;
• memory or concentration problems;
• severe fatigue or breathlessness;
• a sore red tongue with other symptoms;
• neurological symptoms after nitrous oxide use.

Seek urgent help if:

• you are rapidly losing the ability to walk;
• weakness is progressing quickly;
• you develop new bladder or bowel control problems;
• you are severely breathless at rest;
• you have chest pain or collapse;
• you are pregnant and feel significantly unwell.

Call 999 for symptoms of another emergency

Sudden facial weakness, one-sided arm or leg weakness, speech difficulty, severe confusion or sudden vision loss should be treated as possible stroke symptoms rather than assumed to be vitamin deficiency.

Frequently asked questions about vitamin B12 deficiency

Can you have B12 deficiency with a normal full blood count?

Yes. Vitamin B12 deficiency can cause neurological, oral and cognitive symptoms without anaemia, macrocytosis or another obvious full blood count abnormality.

Can you have B12 deficiency with normal haemoglobin?

Yes. Haemoglobin may remain normal, particularly during earlier deficiency or when nerve symptoms are the main presentation.

What does low B12 feel like?

Possible symptoms include fatigue, weakness, dizziness, pins and needles, numbness, a sore tongue, balance problems and difficulty concentrating. These symptoms have many other possible causes.

Can B12 deficiency cause anxiety?

It may contribute to mood or anxiety-like symptoms in some people, but anxiety alone is not specific enough to diagnose deficiency. Other explanations should also be considered.

Can B12 deficiency cause heart palpitations?

Yes, particularly when it causes anaemia. Palpitations may also result from thyroid disease, stress, dehydration, medication or an abnormal heart rhythm.

Can B12 deficiency cause weight gain?

Weight gain is not a typical defining symptom. Reduced activity caused by fatigue might influence weight indirectly, but thyroid, dietary and other metabolic factors are more common explanations.

Can B12 deficiency cause hair loss?

Hair loss is not one of the most specific B12-deficiency symptoms. Iron deficiency, thyroid disease, hormonal changes, stress and several scalp conditions are more common causes.

Can B12 deficiency cause dizziness?

Yes. Dizziness may result from anaemia, weakness or neurological involvement. It can also be caused by low blood pressure, inner-ear problems, dehydration and heart conditions.

Can B12 deficiency cause low blood pressure?

Severe anaemia or significant illness can contribute to faintness and circulatory symptoms, but stable vitamin B12 deficiency does not always lower blood pressure. See our guide to low blood pressure and postural hypotension for other causes.

Does a low-normal B12 result need treatment?

Not always. The decision depends on symptoms, risk factors, the laboratory range and whether additional tests such as MMA are needed. A borderline number should not be interpreted without clinical context.

Can supplements make a B12 test look normal?

Yes. Tablets, injections, multivitamins, fortified drinks and energy products can raise the measured blood level. Tell the clinician what you have taken before testing.

Is methylcobalamin better than cyanocobalamin?

Both can provide vitamin B12. NICE recognises cyanocobalamin, methylcobalamin and adenosylcobalamin as forms that may be used in oral supplements. The most important issue is whether the product provides an adequate dose for the cause of deficiency.

Why is hydroxocobalamin used for injections?

Hydroxocobalamin remains in the body longer than cyanocobalamin and is the injectable form commonly used within the NHS.

How often are B12 injections needed?

The initial and maintenance schedules depend on the cause and whether neurological symptoms are present. People with irreversible malabsorption often require maintenance injections for life.

Can I stop injections when my blood level becomes high?

Not without medical advice. A high measured level after injections is expected and does not mean an irreversible absorption problem has disappeared.

Are vitamin B12 injections dangerous?

They are generally well tolerated. Possible effects include pain or swelling at the injection site, nausea, headache, dizziness or skin reactions. Serious allergic reactions are rare but require emergency help.

Can too much vitamin B12 be harmful?

Vitamin B12 has a wide safety margin, but unnecessary high-dose treatment should not replace proper investigation. A high level in someone not taking supplements can sometimes reflect another medical condition and should be interpreted by a clinician.

How long does it take for B12 tablets to work?

Blood-cell production may begin responding within days, but fatigue and neurological symptoms can take weeks or months to improve.

Do vegans need a B12 supplement?

Vegans need a reliable regular source of vitamin B12 from supplements, fortified foods or both. Unfortified plant foods are not dependable sources.

Can pernicious anaemia be cured?

The autoimmune absorption problem is usually permanent. Vitamin B12 replacement can prevent and treat deficiency, but lifelong treatment is commonly required.

Is pernicious anaemia the same as B12 deficiency?

No. Pernicious anaemia is one cause of B12 deficiency. Deficiency can also result from diet, surgery, bowel disease, medicines and nitrous oxide use.

Can nitrous oxide cause B12 deficiency after one use?

The greatest concern is repeated or heavy use, but susceptibility varies and serious neurological injury has been reported after substantial exposure. New neurological symptoms after use require prompt assessment.

Should folic acid be taken with vitamin B12?

Only when folate deficiency or another clinical reason is present. Vitamin B12 deficiency should be identified and treated first or at the same time so that folic acid does not mask the anaemia while neurological damage continues.