What is the recommended dose of vitamin B12 (cobalamin) for treating deficiency?

Vitamin B12 Dosage for Deficiency Treatment

For confirmed vitamin B12 deficiency, treatment should begin with 1000-2000 μg (1-2 mg) daily oral supplementation or 1000 μg intramuscular injections monthly. 1

Diagnosis Confirmation

Before initiating treatment, confirm B12 deficiency using appropriate testing:

• Initial testing should use either:

  • Total B12 (serum cobalamin) or
  • Active B12 (serum holotranscobalamin)

• Interpretation thresholds:

  • Confirmed deficiency: Total B12 <180 ng/L or active B12 <25 pmol/L
  • Indeterminate: Total B12 180-350 ng/L or active B12 25-70 pmol/L
  • Unlikely deficiency: Total B12 >350 ng/L or active B12 >70 pmol/L 1

• For indeterminate results, measure serum methylmalonic acid (MMA) to confirm deficiency

Treatment Regimens

Oral Supplementation

• Standard oral dosing: 1000-2000 μg daily 1
• Oral supplementation is effective for most patients, including those previously thought to require parenteral therapy (such as pernicious anemia) 1, 2
• Recent evidence shows that oral cyanocobalamin at 1000 μg/day effectively corrects B12 deficiency even in pernicious anemia patients 2

Parenteral (Intramuscular) Supplementation

• Initial loading dose: 30 mcg daily for 5-10 days 3
• Maintenance dose: 100-200 mcg monthly injected intramuscularly 3
• For critically ill patients or those with neurologic disease: Higher doses may be indicated 3
• For pernicious anemia: Intramuscular therapy has traditionally been recommended for life 3

Special Populations

• Post-bariatric surgery: 1000 μg oral B12 daily indefinitely 1
• Ileal resection >20 cm: 1000 mcg vitamin B12 monthly indefinitely 1
• Crohn's disease with ileal involvement/resection: 1000 μg monthly if >20 cm of distal ileum is resected 1
• Vegans and vegetarians: 250-350 μg daily or 1000 μg weekly 1
• Children: 1-5 mg over 2+ weeks in doses of 100 mcg, then 30-50 mcg every 4 weeks for maintenance 3

Route Selection Considerations

• Oral route advantages:

  • Better patient compliance
  • Cost-effectiveness
  • Suitable for patients on anticoagulants or with needle phobia 1
  • Recent evidence supports efficacy even in pernicious anemia 2

• Intramuscular route preferred for:

  • Severe neurological symptoms
  • Malabsorption disorders
  • Critically ill patients requiring rapid correction 1
  • According to British National Formulary guidelines, 1000 μg intramuscular hydroxocobalamin once every two months is sufficient for maintenance after initial loading 4

Monitoring Response

• Check B12 levels after 3 months of supplementation 1
• If B12 levels remain indeterminate (180-350 ng/L), measure MMA levels 1
• Monitor for clinical improvement:
  • Median time to reverse initial vitamin B12 deficiency abnormalities ranges from 1 month for hemolysis to 4 months for mucosal symptoms 2
• Monitor serum potassium closely during the first 48 hours of treatment and administer potassium if necessary 3

Important Caveats

• Untreated B12 deficiency for more than 3 months may cause permanent degenerative lesions of the spinal cord 1
• Treatment should be based on clinical symptoms rather than solely on biomarker levels 4
• Up to 50% of individuals may require individualized injection regimens with more frequent administration (ranging from daily or twice weekly to every 2-4 weeks) to remain symptom-free 4
• Patients taking metformin require monitoring of B12 levels due to increased risk of deficiency 1
• The lowest effective oral dose of cyanocobalamin to normalize mild B12 deficiency is significantly higher than the recommended dietary allowance of approximately 3 μg daily 5