What are the instructions for intramuscular (IM) injection of methylcobalamin (Vitamin B12)?

Intramuscular Injection Instructions for Methylcobalamin

For vitamin B12 deficiency with neurological symptoms, administer methylcobalamin 1 mg (1000 mcg) intramuscularly on alternate days until neurological improvement plateaus, then transition to 1 mg IM every 2 months for lifelong maintenance. 1, 2

Initial Loading Phase Dosing

For patients with neurological involvement:

• Administer 1 mg (1000 mcg) IM on alternate days until no further neurological improvement occurs 1, 2
• This aggressive initial regimen is critical because neurological complications can become irreversible if undertreated 2

For patients without neurological involvement:

• Administer 1 mg IM three times per week for 2 weeks 1
• Then transition to maintenance dosing 1

Maintenance Phase Protocol

• After neurological improvement plateaus, administer 1 mg IM every 2-3 months for life 1, 2
• Monthly dosing of 1000 mcg IM is an acceptable alternative that may be necessary to meet metabolic requirements in some patients 1
• Never discontinue therapy even if symptoms resolve, as deficiency will recur without ongoing supplementation 2

Injection Site Selection and Technique

Preferred injection sites:

• Deltoid muscle (upper arm) or vastus lateralis (lateral thigh) are preferred 1
• Avoid the buttock as a routine injection site due to potential sciatic nerve injury risk 1
• If buttock must be used, only inject in the upper outer quadrant with the needle directed anteriorly 1

For patients with thrombocytopenia:

• Standard IM administration can be safely performed if platelet count >50 × 10⁹/L 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 <25 × 10⁹/L with neurological symptoms, prioritize treatment despite low platelets 1
• Consider platelet transfusion support if platelet count <10 × 10⁹/L 1
• Monitor injection sites for hematoma formation 1

Monitoring Requirements

• Check serum B12 and homocysteine every 3 months until stabilization, then annually 1, 2
• Target homocysteine level <10 μmol/L for optimal neurological outcomes 1, 2
• Evaluate for resolution of neurological symptoms such as paresthesias, gait disturbances, or cognitive changes 1

Critical Pitfalls to Avoid

Never administer folic acid before treating B12 deficiency, as it may mask the deficiency while allowing neurological damage to progress, potentially precipitating subacute combined degeneration of the spinal cord 1, 2

Do not stop therapy when symptoms improve or B12 levels normalize—lifelong maintenance is required unless the underlying cause is definitively corrected 2

Special Considerations

Choice of B12 formulation:

• Methylcobalamin or hydroxocobalamin may be preferable to cyanocobalamin in patients with renal dysfunction, as cyanocobalamin requires renal clearance of the cyanide moiety and is associated with increased cardiovascular events (HR 2.0) in diabetic nephropathy 1, 3
• Both methylcobalamin and adenosylcobalamin are essential with distinct metabolic functions; methylcobalamin is primarily involved in hematopoiesis and brain development, while adenosylcobalamin affects myelin formation 4

Post-bariatric surgery patients:

• Require 1 mg every 3 months IM or 1 mg daily orally 1

Patients with ileal resection >20 cm:

• Should receive prophylactic 1000 mcg IM monthly for life 1