How many International Units (IU) of vitamin B12 should be administered as a weekly intramuscular injection for an adult with documented vitamin B12 deficiency?

Weekly Vitamin B12 Injection Dosing

For weekly intramuscular vitamin B12 injections in adults with documented deficiency, administer 1000 mcg (1 mg) of hydroxocobalamin once per week. 1

Standard Treatment Protocols by Clinical Scenario

Deficiency WITHOUT Neurological Symptoms

• Initial loading phase: Hydroxocobalamin 1000 mcg IM three times weekly for 2 weeks 1, 2
• Maintenance phase: Transition to 1000 mcg IM every 2–3 months for life 1, 3
• If converting standard maintenance to weekly dosing, use 1000 mcg IM weekly 1

Deficiency WITH Neurological Symptoms

• Intensive initial treatment: Hydroxocobalamin 1000 mcg IM on alternate days until neurological improvement plateaus (may require weeks to months) 1, 3
• Neurological manifestations include paresthesias, numbness, gait disturbances, cognitive difficulties, memory problems, and glossitis 1
• After neurological recovery: Maintain with 1000 mcg IM every 2 months for life 1
• Critical warning: Treatment delays can cause irreversible subacute combined degeneration of the spinal cord 3

Post-Bariatric Surgery Patients

• Prophylactic regimen: Hydroxocobalamin 1000 mcg IM every 3 months indefinitely, regardless of documented deficiency 1
• Alternative oral dosing: 1000–2000 mcg daily orally 3
• Women planning pregnancy: Check serum B12 every 3 months throughout conception and gestation 1, 3

Why 1000 mcg (Not Lower Doses)

The 1000 mcg dose is superior to 100 mcg injections because significantly greater amounts of vitamin are retained with the higher dose, with no disadvantage in cost or toxicity. 4 Monthly maintenance using 1000 mcg may be necessary to meet metabolic requirements in many patients, and up to 50% of individuals require more frequent administration (ranging from twice weekly to every 2–4 weeks) to remain symptom-free. 5, 4

Preferred Formulation: Hydroxocobalamin vs. Cyanocobalamin

Hydroxocobalamin is the guideline-recommended first-line injectable due to superior tissue retention and established dosing protocols across all major medical societies. 1

Avoid Cyanocobalamin in Renal Dysfunction

• Contraindicated in patients with estimated GFR <50 mL/min because cyanocobalamin requires renal clearance of its cyanide moiety 1, 3
• In diabetic nephropathy, cyanocobalamin doubled cardiovascular event risk (hazard ratio ≈2.0) compared to placebo 1
• Use hydroxocobalamin or methylcobalamin instead in renal impairment 1, 3

Critical Safety Precautions

Folate Interaction

Never administer folic acid before correcting vitamin B12 deficiency. 1, 3, 2 Folic acid can mask megaloblastic anemia while allowing irreversible subacute combined degeneration of the spinal cord to progress. 1, 3 Only add folic acid 5 mg daily after B12 repletion if folate deficiency is documented. 1

Lifelong Treatment Requirements

• Do not discontinue B12 supplementation even if levels normalize in patients with malabsorption (pernicious anemia, ileal resection >20 cm, post-bariatric surgery) 1, 2
• These patients require lifelong therapy and can relapse if treatment is stopped 1, 3

Monitoring Schedule

First Year

• Recheck serum B12 at 3 months, 6 months, and 12 months 1, 3
• At each visit, measure: serum B12, complete blood count, methylmalonic acid (if B12 remains borderline), and homocysteine 1
• Target homocysteine <10 μmol/L for optimal cardiovascular outcomes 1, 2

After Stabilization

• Annual monitoring once levels stabilize for two consecutive checks 1, 3
• Measure serum B12 directly before the next scheduled injection (trough level) to identify potential under-dosing 1

Post-Bariatric Surgery Patients

• Check B12 every 3 months if planning pregnancy 1, 3
• Monitor additional micronutrients (vitamin D ≥75 nmol/L, thiamine, calcium, vitamin A) at least every 6 months 1

Common Pitfalls to Avoid

• Do not rely solely on serum B12 to rule out deficiency—standard testing misses functional deficiency in up to 50% of cases 3
• Do not stop monitoring after one normal result—patients with malabsorption require ongoing assessment 1, 3
• Do not use "titration" of injection frequency based on biomarkers—clinical response and symptom resolution should guide treatment adjustments 5
• Evaluate for concurrent deficiencies (iron, zinc, copper, selenium) if anemia or fatigue persists despite adequate B12 repletion 1, 2

Alternative to Weekly Injections

High-dose oral vitamin B12 (1000–2000 mcg daily) is as effective as intramuscular administration for correcting deficiency in most patients, regardless of etiology. 3, 6, 7, 8 However, intramuscular therapy leads to more rapid improvement and should be considered in patients with severe deficiency or severe neurologic symptoms. 3, 6, 7