Vitamin B12 Injection Dosing for Pernicious Anemia
For maintenance therapy in pernicious anemia, administer 100 mcg intramuscularly every month for life, or alternatively, 1000 mcg intramuscularly every 2-3 months for life. 1, 2
Initial Loading Phase
Before starting maintenance therapy, patients require an initial loading regimen:
- Administer 100 mcg daily for 6-7 days by intramuscular or deep subcutaneous injection 1
- If clinical improvement and reticulocyte response occur, give the same dose on alternate days for seven doses 1
- Then continue every 3-4 days for another 2-3 weeks until hematologic values normalize 1
Maintenance Therapy Options
After the loading phase, two evidence-based maintenance regimens exist:
FDA-Approved Standard Regimen
- 100 mcg intramuscularly once monthly for life 1
- This is the FDA-labeled dosing for pernicious anemia 1
Alternative Higher-Dose Regimen
- 1000 mcg (1 mg) intramuscularly every 2-3 months for life 2, 3
- This regimen is supported by current clinical guidelines and may be necessary to meet metabolic requirements in many patients 4
- The higher dose provides greater vitamin retention with no disadvantage in cost or toxicity 4
Special Considerations for Neurological Involvement
If neurological symptoms are present at diagnosis:
- Administer hydroxocobalamin 1 mg intramuscularly on alternate days until no further improvement 2, 3
- Then transition to maintenance with 1 mg intramuscularly every 2 months for life 2, 3
- The more aggressive initial dosing is critical because neurological sequelae may become irreversible despite later cobalamin replacement 5
Oral Alternative (If Patient Prefers)
While intramuscular therapy is the gold standard, oral therapy is an acceptable alternative:
- 1000 mcg cyanocobalamin orally daily 6, 7, 4
- This achieves adequate absorption through passive enteral absorption (1-2% of dose) despite intrinsic factor deficiency 6
- A recent 2024 prospective cohort study demonstrated that 88.5% of pernicious anemia patients were no longer deficient after 1 month of oral supplementation at this dose 7
- Oral therapy requires patient adherence and may need more frequent monitoring initially 6
Critical Pitfalls to Avoid
- Never use the intravenous route - almost all vitamin will be lost in urine 1
- Never administer folic acid before treating B12 deficiency - this may mask the deficiency and precipitate subacute combined degeneration of the spinal cord 3
- Never discontinue therapy even if levels normalize - pernicious anemia requires lifelong treatment 2, 3
- Avoid cyanocobalamin in patients with renal dysfunction - use methylcobalamin or hydroxocobalamin instead due to potential cyanide accumulation 2, 3