What is the recommended dosage and frequency of mecobalamin (Vitamin B12) injection for treating megaloblastic anemia?

Mecobalamin (Vitamin B12) Injection Dosage for Megaloblastic Anemia

For megaloblastic anemia due to vitamin B12 deficiency, administer hydroxocobalamin 1 mg intramuscularly three times weekly for 2 weeks, followed by lifelong maintenance of 1 mg intramuscularly every 2-3 months. 1, 2

Initial Treatment Phase

Without Neurological Involvement

• Hydroxocobalamin 1 mg intramuscularly three times per week for 2 weeks 1, 2
• This loading phase ensures rapid correction of the deficiency and allows for reticulocyte response, which typically occurs between days 5-10 of treatment 3
• The FDA label for cyanocobalamin suggests 100 mcg daily for 6-7 days, then alternate days for seven doses, but current guidelines favor the higher 1 mg dose with hydroxocobalamin 4, 1

With Neurological Involvement

• Hydroxocobalamin 1 mg intramuscularly on alternate days until no further neurological improvement occurs 1, 2
• This more aggressive regimen is critical because neurological damage from B12 deficiency can become irreversible if not treated promptly 1
• Neurological symptoms include peripheral neuropathy, cognitive impairment, loss of vibration sense, and subacute combined degeneration of the spinal cord 3, 1

Maintenance Therapy

Hydroxocobalamin 1 mg intramuscularly every 2-3 months for life 1, 2

• Some patients may require monthly dosing (1000 mcg IM) to meet metabolic requirements, particularly if symptoms recur on the standard schedule 1, 5
• Lifelong treatment is necessary because the underlying cause (typically malabsorption from pernicious anemia, gastric surgery, or ileal disease) cannot be reversed 1, 2

Critical Considerations

Route of Administration

• Intramuscular or deep subcutaneous injection is mandatory for patients with malabsorption 2, 4
• The FDA label explicitly warns against intravenous administration, as almost all vitamin will be lost in urine 4
• While oral high-dose B12 (1-2 mg daily) can be effective for correcting anemia, intramuscular therapy leads to more rapid improvement and should be used for severe deficiency or neurologic symptoms 6

Folic Acid Warning

Never administer folic acid before treating vitamin B12 deficiency, as it may mask B12 deficiency and precipitate subacute combined degeneration of the spinal cord 1, 2

• Always check both B12 and folate levels, as deficiencies may coexist 2
• If both are deficient, treat B12 first or simultaneously, never folate alone 2, 4

Monitoring Response

• Reticulocytosis should appear between days 5-10 of treatment 3
• Hemoglobin, mean corpuscular volume, white blood cell count, and platelet count should normalize by day 30-90 3
• Neurological improvement, when present, typically occurs within the first month of treatment 3
• After stabilization, check serum B12 levels and homocysteine every 3 months until stable, then annually 1

Special Populations

Post-Bariatric Surgery

• 1 mg intramuscularly monthly for life OR 1000-2000 mcg daily sublingual 1
• Patients with more than 20 cm of distal ileum resected require prophylactic monthly injections indefinitely 1

Thrombocytopenia

• For platelet counts >50 × 10⁹/L, standard IM administration is safe 1
• For platelet counts 25-50 × 10⁹/L, use smaller gauge needles (25-27G) and apply prolonged pressure (5-10 minutes) at injection site 1
• For platelet counts <10 × 10⁹/L, consider platelet transfusion support before IM administration 1

Common Pitfalls to Avoid

• Do not use the 100 mcg dose recommended in older FDA labels—current evidence supports 1000 mcg (1 mg) for both loading and maintenance 1, 5
• Do not discontinue therapy even if levels normalize—patients with malabsorption require lifelong treatment 1, 2
• Do not delay treatment in patients with neurological symptoms—irreversible damage can occur 1, 2
• Do not assume oral therapy is adequate for megaloblastic anemia—parenteral administration is preferred for initial treatment and mandatory for malabsorption 2, 6