What is the recommended dose for vitamin B12 (cobalamin) injections given weekly?

Vitamin B12 Injection Dosing Recommendations

For vitamin B12 deficiency, the recommended weekly injection dose is 1000 mcg (1 mg) of vitamin B12 intramuscularly. 1

Treatment Protocols Based on Clinical Scenario

Initial Loading Dose

• For patients with clinical B12 deficiency: 1000 mcg intramuscularly every other day for 1-2 weeks 2, 1
• Alternative loading regimen: 1000 mcg intramuscularly 5-6 times biweekly 3

Maintenance Therapy

• Standard maintenance: 1000 mcg intramuscularly monthly, indefinitely 2, 1
• For patients with ileal resection >20 cm: 1000 mcg intramuscularly monthly for life 2
• For some patients: More frequent administration may be needed (every 2-4 weeks) to remain symptom-free 4

Special Considerations

Route of Administration

• Intramuscular route: Preferred for patients with:

  • Severe deficiency
  • Neurological symptoms
  • Malabsorption issues
  • Ileal resection >20 cm 2, 5

• Oral alternative: High-dose oral supplementation (1000-2000 mcg daily) may be effective for most patients without severe neurological involvement or malabsorption 1, 6

  • However, parenteral supplementation remains the reference standard, especially for Crohn's disease patients with ileal resection 2

Monitoring Response

• Assess response after 3 months by measuring serum B12 levels
• Monitor platelet count until normalization
• Periodically assess B12 levels during maintenance therapy 1

Important Cautions

• Do not administer folic acid before treating B12 deficiency as it may mask underlying deficiency and precipitate subacute combined degeneration of the spinal cord 1, 7
• Patients with pernicious anemia require monthly injections for life; failure to maintain treatment will result in return of anemia and irreversible neurological damage 7
• The 3-monthly injection schedule previously advised is now considered insufficient; monthly administration is necessary to prevent clinical manifestations of deficiency 2

Patient Selection for B12 Testing

• Testing is warranted in patients with risk factors:
  • Gastric or small intestine resections
  • Inflammatory bowel disease (especially Crohn's with ileal involvement)
  • Metformin use >4 months
  • Proton pump inhibitor use >12 months
  • Vegans or strict vegetarians
  • Adults >75 years 5, 8

The evidence strongly supports using 1000 mcg weekly doses for initial treatment of vitamin B12 deficiency, with transition to monthly maintenance therapy for most patients. While oral therapy shows promise, intramuscular administration remains the gold standard, especially for patients with malabsorption or neurological symptoms.