For an adult with documented vitamin B12 deficiency requiring a weekly intramuscular injection, how many milliliters should be drawn from a vial that contains 1000 µg (1 mg) per mL?

Vitamin B12 Injection Dosing: Standard Volume from 1000 mcg/mL Vial

For an adult with documented vitamin B12 deficiency requiring weekly intramuscular injection, draw 1 mL (1000 mcg) from the vial containing 1000 mcg per mL. 1

Standard Dosing Protocol

The guideline-recommended dose for vitamin B12 intramuscular therapy is 1000 mcg (1 mg) per injection, regardless of the clinical scenario. 1 When your vial contains 1000 mcg/mL, this translates directly to 1 mL per injection. 1

Dosing by Clinical Context

For deficiency without neurological symptoms:

• Initial loading: 1000 mcg (1 mL) IM three times weekly for 2 weeks 1
• Maintenance: 1000 mcg (1 mL) IM every 2–3 months for life 1

For deficiency with neurological involvement (paresthesias, gait disturbance, cognitive changes, glossitis):

• Intensive phase: 1000 mcg (1 mL) IM on alternate days until neurological improvement plateaus 1
• Maintenance: 1000 mcg (1 mL) IM every 2 months for life 1

For post-bariatric surgery patients:

• Prophylactic regimen: 1000 mcg (1 mL) IM every 3 months indefinitely 1, 2
• Alternative: 1000 mcg (1 mL) IM monthly for life 1

For high-risk populations (ileal resection >20 cm, Crohn's disease with ileal involvement):

• Prophylactic: 1000 mcg (1 mL) IM monthly for life, even without documented deficiency 1

Why 1000 mcg Is the Standard Dose

The 1000 mcg dose provides superior tissue retention compared to lower doses. 3 Studies demonstrate that significantly greater amounts of vitamin B12 are retained with a 1000 mcg injection than with 100 mcg, with no disadvantage in cost or toxicity. 3 Monthly dosing of 1000 mcg IM is an acceptable alternative to every 2–3 months and may better meet metabolic requirements in some patients, particularly those with persistent symptoms despite standard dosing, post-bariatric surgery patients, or patients with extensive ileal disease. 1

Preferred Formulation

Hydroxocobalamin is the guideline-recommended first-line injectable for adult vitamin B12 deficiency, with the standard 1000 mcg dose administered intramuscularly. 1 In patients with renal dysfunction (estimated GFR <50 mL/min), avoid cyanocobalamin and use hydroxocobalamin or methylcobalamin instead, because cyanocobalamin requires renal clearance of the cyanide moiety and is associated with increased cardiovascular events (hazard ratio ≈2.0 in diabetic nephropathy). 1

Critical Safety Considerations

Never administer folic acid before ensuring adequate B12 treatment, as folic acid can mask megaloblastic anemia while allowing irreversible subacute combined degeneration of the spinal cord to progress. 1, 2 After B12 repletion, add folic acid 5 mg daily only if folate deficiency is documented. 1

Prompt correction is essential in patients with neurological manifestations because treatment delays can lead to irreversible neurological damage. 2 Aggressive alternate-day intramuscular dosing is mandatory when neurological symptoms are present to achieve timely functional recovery. 1

Practical Administration

• Use a 23–25 gauge needle for standard IM injection 1
• Preferred sites: deltoid or vastus lateralis muscle 1
• Avoid the buttock as a routine injection site due to potential sciatic nerve injury risk; if used, only the upper outer quadrant should be used with the needle directed anteriorly 1
• For patients with severe thrombocytopenia (platelet count 25–50 × 10⁹/L), use smaller gauge needles (25–27G) and apply prolonged pressure (5–10 minutes) at the injection site 1

Monitoring Schedule

• Recheck serum B12 at 3 months, 6 months, and 12 months in the first year 1, 2
• Annual monitoring once levels stabilize 1
• At follow-up, measure serum B12, complete blood count, and target homocysteine <10 μmol/L for optimal cardiovascular outcomes 1
• For post-bariatric surgery patients planning pregnancy, check B12 levels every 3 months 1, 2