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Vitamin B12 deficiency

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"Vitamin B 12 Deficiency"[MeSH Terms]

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Vitamin B12 deficiency NIH fact sheet symptoms diagnosis treatment

Quick answer

Vitamin B12 deficiency occurs when the body lacks enough cobalamin, a vitamin needed for normal red blood cell production, nerve function, and DNA synthesis.

Common symptoms

  • Tiredness, weakness, shortness of breath, palpitations
  • Pale skin or mild jaundice
  • Sore, red tongue or mouth ulcers
  • Tingling, numbness, burning sensations, or “pins and needles” in hands/feet
  • Poor balance, walking difficulty, muscle weakness
  • Memory problems, low mood, irritability, or confusion
  • In infants/children: poor growth or developmental delay
Neurological symptoms can occur even without anemia.

Frequent causes

  • Low dietary intake, especially strict vegan diets without supplementation
  • Poor absorption due to pernicious anemia, an autoimmune condition affecting intrinsic factor
  • Stomach or intestinal surgery, including bariatric surgery
  • Coeliac disease, Crohn disease, or other intestinal disorders
  • Medicines such as metformin or long-term acid-suppressing drugs
  • Increased risk with older age

Diagnosis

A clinician usually checks:
  • Full blood count, which may show macrocytic anemia
  • Serum vitamin B12
  • Sometimes methylmalonic acid or homocysteine if results are unclear
  • Tests for pernicious anemia, such as intrinsic-factor antibodies, when appropriate

Treatment

Treatment depends on the cause and severity:
  • Dietary deficiency: oral B12 supplements and regular B12-fortified foods or supplements long term if dietary intake remains low.
  • Absorption problems or neurological symptoms: B12 injections are commonly used initially, followed by maintenance injections or high-dose oral therapy depending on local guidance and cause.
  • The underlying cause should be identified, especially pernicious anemia, which generally requires lifelong replacement.
Do not self-treat prolonged numbness, balance changes, or memory symptoms alone. Nerve injury from untreated deficiency may become permanent. Seek urgent medical assessment for severe weakness, confusion, marked breathlessness, chest pain, or rapidly worsening neurological symptoms.
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https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessio…

Vitamin B12 deficiency

Vitamin B12, also called cobalamin, is needed for normal red-blood-cell formation, DNA synthesis, and nerve function. Deficiency can cause megaloblastic anemia and neurological injury, sometimes even when the blood count is normal. NIH B12 fact sheet

Common symptoms

  • Tiredness, weakness, pallor, breathlessness, palpitations, dizziness
  • Sore, smooth red tongue (glossitis), reduced appetite, diarrhea or weight loss
  • Pins-and-needles, numbness, burning sensations, poor balance or difficulty walking
  • Memory problems, slowed thinking, irritability, low mood, or confusion
Nerve symptoms can occur without anemia, and may not fully reverse if diagnosis and treatment are delayed. NHLBI overview

Important causes

  1. Poor absorption
    • Pernicious anemia: autoimmune loss of intrinsic factor
    • Gastric surgery or bariatric surgery
    • Atrophic gastritis, low stomach acid
    • Crohn disease or disease/resection of the terminal ileum
    • Coeliac disease, bacterial overgrowth, pancreatic disease
  2. Low dietary intake
    • Strict vegan diet without fortified foods or supplementation
    • Malnutrition or low intake of animal-source foods
  3. Medicines/exposures
    • Metformin
    • Long-term proton-pump inhibitors or H2 blockers
    • Nitrous oxide exposure or misuse
Goldman-Cecil Medicine, section “Cobalamin (Vitamin B12) Deficiency,” lines 1064-1089.

How it is diagnosed

A clinician usually considers:
  • Full blood count and blood film: anemia with high MCV, macro-ovalocytes, hypersegmented neutrophils
  • Serum B12 level
  • If B12 is borderline or symptoms strongly suggest deficiency: methylmalonic acid (MMA) and often homocysteine
  • Folate, iron studies, thyroid/liver tests as appropriate
  • Anti-intrinsic-factor antibody testing if pernicious anemia is suspected
A low serum B12 supports the diagnosis, but a normal result does not always exclude functional deficiency. Goodman & Gilman’s The Pharmacological Basis of Therapeutics, p. 933.

Treatment

Treatment depends on the cause and whether neurological symptoms are present:
  • Dietary deficiency: oral vitamin B12 supplementation plus dietary advice.
  • Malabsorption, pernicious anemia, or significant neurological symptoms: B12 injections are often used initially, followed by maintenance treatment. Some causes require lifelong replacement.
  • Do not treat possible B12 deficiency with folic acid alone, because it can improve anemia while neurological damage continues.
The exact dose, route, and duration should be set by a clinician based on tests, cause, severity, pregnancy status, and local guidance.

Foods containing B12

Naturally found mainly in meat, fish, shellfish, eggs, milk, and dairy foods. Vegans generally need reliable fortified foods or a B12 supplement.

Seek prompt medical care if

You have new numbness, weakness, balance trouble, difficulty walking, confusion, severe fatigue, chest pain, fainting, or shortness of breath. These can indicate significant anemia or nerve involvement.
If you share your symptoms, diet, current medicines, and any blood-test values such as B12, hemoglobin, and MCV, I can help interpret what they may mean.
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