What is the recommended dosing regimen for Vitamin B12 (cobalamin) deficiency in adults?

Vitamin B12 Dosing for Deficiency in Adults

For vitamin B12 deficiency in adults, the recommended treatment is 1,000 mcg cyanocobalamin IM daily for days 1-10, followed by 1,000 mcg IM monthly for maintenance therapy in cases of pernicious anemia or severe malabsorption, while oral therapy at 1,500-2,000 mcg daily is equally effective for most other patients. 1

Treatment Approach Based on Deficiency Cause

Pernicious Anemia

• Parenteral (IM) administration is traditionally recommended:
  • Loading dose: 1,000 mcg cyanocobalamin IM daily for 7-10 days
  • Maintenance: 1,000 mcg IM monthly for life 2
  • Folic acid should be administered concomitantly if needed

Normal Intestinal Absorption

• Oral administration (preferred):
  • 1,500-2,000 mcg daily for 3 months 1
  • Absorption rates of 1-2% occur via passive diffusion even in patients with malabsorption
  • Sublingual B12 supplementation offers comparable efficacy to IM administration with better patient compliance 1

Malabsorption Conditions

• For mild to moderate malabsorption:

  • Oral dose: 1,000-2,000 mcg daily 1, 3
  • Recent evidence shows oral supplementation with 1,000 mcg/day of cyanocobalamin effectively treats vitamin B12 deficiency even in pernicious anemia 3

• For severe malabsorption (e.g., ileal resection >30 cm):

  • IM administration may be preferred: 1,000 mcg monthly 1, 4
  • Some patients may require individualized injection regimens (every 2-4 weeks) to remain symptom-free 4

Monitoring and Response Assessment

• Initial monitoring:

  • Assess clinical response and reticulocyte count after 1 week
  • Monitor platelet count until normalization 1

• Follow-up monitoring:

  • Measure serum B12 levels after 3 months of treatment 1
  • For long-term therapy, periodic assessment of B12 levels is recommended

Special Considerations

• High-risk populations requiring more aggressive screening and earlier intervention:

  • Elderly patients (>75 years)
  • Patients on metformin (>4 months)
  • Patients on proton pump inhibitors (>12 months)
  • Vegans or strict vegetarians
  • Patients with malabsorption disorders
  • Patients with gastric or small intestine resections 1

• Post-bariatric surgery patients:

  • Require 1 mg oral vitamin B12 daily indefinitely 5

Important Caveats

• Oral vs. IM administration:

  • High-dose oral therapy (1,000-2,000 mcg daily) is as effective as IM administration for most patients 1, 5, 6
  • IM therapy leads to more rapid improvement and should be considered in patients with severe deficiency or severe neurologic symptoms 5
  • The effective oral dose is significantly higher than the recommended dietary allowance of approximately 3 mcg daily 7

• Diagnostic pitfalls:

  • Serum B12 testing alone has limitations; consider methylmalonic acid (MMA) levels, homocysteine levels, and clinical symptoms 1
  • Some patients may have functional B12 deficiency despite normal serum levels 1

• Treatment duration:

  • For reversible causes: supplementation until cause is addressed
  • For irreversible causes (pernicious anemia): lifelong supplementation 1
  • Untreated vitamin B12 deficiency may cause permanent degenerative lesions of the spinal cord 1