Vitamin B12 deficiency: signs, causes and who is at risk

Short answer: Vitamin B12 is a nutrient the body cannot make on its own, yet it is essential for red blood cell formation, nervous system health, and DNA synthesis. A deficiency can develop slowly and may cause symptoms ranging from fatigue to neurological changes, but it is often preventable with the right diet or supplementation under medical guidance.

What does vitamin B12 actually do?

According to the NIH Office of Dietary Supplements (ODS), vitamin B12 serves three core physiological roles:

• It is required for healthy red blood cell formation.
• It is required for the development, myelination, and function of the central nervous system.
• It is required for DNA synthesis.

When the body does not get enough B12 over time, each of these processes can be affected, which is why the symptoms of deficiency can appear in seemingly unrelated parts of the body.

How much vitamin B12 do adults need?

The NIH ODS sets the adult daily target for vitamin B12 at 2.4 micrograms (mcg). No Tolerable Upper Intake Level has been established because of its low potential for toxicity. The body does not store excess amounts and absorption is self-limiting at high oral doses, meaning the body absorbs less as the dose increases. This makes vitamin B12 generally considered safe even at higher doses, though population reference values like the 2.4 mcg target are not personal prescriptions. Always consult a clinician or pharmacist to determine the right amount for your individual situation.

Signs and symptoms of deficiency

Because B12 is involved in red blood cell production and nervous system function, deficiency can produce a wide range of symptoms. These may develop gradually and are not unique to B12 deficiency, so a blood test is the only reliable way to confirm low levels. Common signs include:

• Fatigue and weakness
• Pale or jaundiced skin
• Shortness of breath and dizziness
• Numbness or tingling in the hands and feet
• Difficulty walking or balance problems
• Mouth soreness or a swollen, inflamed tongue
• Mood changes, memory difficulties, or cognitive fog

One well-established consequence of prolonged deficiency is megaloblastic anemia, a condition in which the body produces abnormally large, poorly functioning red blood cells. The evidence that adequate vitamin B12 helps prevent megaloblastic anemia is graded as strong by the NIH ODS. If you recognise any of these symptoms, speak to a doctor before self-treating.

What causes vitamin B12 deficiency?

Insufficient dietary intake

Vitamin B12 is found almost exclusively in animal-derived foods such as meat, fish, poultry, eggs, and dairy. People who follow a vegan or strict vegetarian diet are therefore at higher risk of not meeting the 2.4 mcg daily target through food alone. Fortified foods and supplements are the primary alternatives for this group.

Poor absorption

Even when dietary intake is adequate, the body may struggle to absorb B12 properly. Absorption requires a protein called intrinsic factor, produced in the stomach. Conditions that reduce stomach acid or damage the stomach lining can therefore limit how much B12 enters the bloodstream. These include:

• Pernicious anemia, an autoimmune condition that destroys intrinsic factor-producing cells
• Atrophic gastritis, a thinning of the stomach lining common in older adults
• Gastrointestinal surgeries such as gastric bypass
• Certain digestive disorders including Crohn's disease and celiac disease

Medication interactions

Several commonly prescribed medications can interfere with B12 absorption. The NIH ODS highlights the following interactions:

• Proton pump inhibitors (PPIs) such as omeprazole (Prilosec) and lansoprazole (Prevacid) can interfere with vitamin B12 absorption from food by slowing gastric acid release, and may lead to vitamin B12 deficiency.
• Histamine H2-receptor antagonists such as cimetidine (Tagamet) and ranitidine (Zantac) can similarly interfere with vitamin B12 absorption from food.
• Metformin, an antihyperglycemic used for prediabetes and type 2 diabetes, might reduce vitamin B12 absorption and significantly lower serum vitamin B12 concentrations.

If you take any of these medications, talk to your doctor before adding a vitamin B12 supplement, and ask whether routine monitoring of your B12 levels is appropriate.

Who is most at risk?

Certain groups are more likely to have low vitamin B12 levels and may benefit from closer monitoring or dietary attention:

• Older adults - atrophic gastritis becomes more common with age, reducing the ability to absorb B12 from food. Crystalline B12 in fortified foods and supplements is absorbed more readily than B12 bound to food protein, making these sources particularly useful for this group.
• People following vegan or vegetarian diets - animal products are the primary natural source of B12, so those who avoid them need reliable fortified food sources or supplements.
• People with gastrointestinal conditions - disorders affecting the stomach or small intestine can impair absorption regardless of dietary intake.
• People who have had bariatric surgery - procedures that alter or remove parts of the stomach can reduce intrinsic factor production.
• People taking metformin, PPIs, or H2 blockers - as outlined above, these medications can reduce B12 absorption over time.
• Pregnant and breastfeeding individuals - B12 needs increase during pregnancy and lactation; deficiency in a mother can affect fetal and infant neurological development.

What the evidence does and does not support

It is important to be clear about what vitamin B12 supplementation is and is not proven to do. The NIH ODS grades the evidence as follows:

• B12 is required for red blood cell formation, nervous system function, and DNA synthesis: strong evidence.
• Adequate B12 helps prevent megaloblastic anemia: strong evidence.
• B12 supplementation reduces the risk of cardiovascular disease or stroke: insufficient evidence.
• B12 supplementation improves cognitive function or slows dementia or Alzheimer's disease: insufficient evidence.
• B12 supplementation boosts energy, athletic performance, or endurance in people who are not deficient: insufficient evidence.
• B12 intake or blood level affects cancer risk: insufficient evidence.

Vitamin B12 supplements are not intended to diagnose, treat, cure or prevent any disease, and these statements have not been evaluated by the FDA. Supplements may help correct a confirmed deficiency, but they are not a substitute for medical evaluation.

When to see a doctor

If you have symptoms that could suggest B12 deficiency, a simple blood test can help clarify your status. You should speak to a healthcare provider if you are in one of the at-risk groups listed above, if you take medications known to affect B12 absorption, or if you are pregnant or planning to become pregnant. Do not adjust your supplement intake or stop prescribed medications without professional guidance.

Sources: NIH Office of Dietary Supplements, Vitamin B12 Fact Sheet for Health Professionals and Consumer versions. Available at ods.od.nih.gov.