What is the appropriate clinical dose of vitamin B12 for a 73-year-old patient with B12‑deficiency anemia who needs weekly intramuscular injections until gastroenterology follow‑up?

Recommended Vitamin B12 Dose for Weekly Intramuscular Injections

Administer hydroxocobalamin 1000 µg (1 mg) intramuscularly once weekly for 4–8 weeks until the gastroenterology appointment, then transition to maintenance therapy based on the underlying cause identified by GI evaluation. 1

Initial Treatment Protocol

For this 73-year-old patient with B12-deficiency anemia awaiting GI evaluation, the standard loading regimen should be modified to accommodate weekly clinic visits:

• Give 1000 µg hydroxocobalamin intramuscularly once weekly for the 4–8 week period until GI specialty consultation 1, 2
• This weekly schedule is a practical adaptation of the guideline-recommended intensive regimen (three times weekly for 2 weeks when no neurological symptoms are present) 1
• The 1000 µg dose is the established standard across all major guidelines and provides adequate tissue saturation 1, 3

Why This Dose and Schedule

• Hydroxocobalamin 1000 µg is the guideline-endorsed dose for all B12 deficiency treatment, whether given three times weekly, weekly, or monthly 1, 2
• Monthly dosing of 1000 µg IM is more effective than lower doses and may be necessary to meet metabolic requirements in many patients 2, 3
• The larger 1000 µg injection results in significantly greater vitamin retention compared to 100 µg doses, with no disadvantage in cost or toxicity 3

Critical Considerations for This Patient

The combination of B12 deficiency AND iron deficiency strongly suggests gastrointestinal blood loss, making the GI referral urgent and appropriate 1:

• Patients with ileal disease (Crohn's), ileal resection >20 cm, or pernicious anemia will require lifelong monthly injections 1, 4
• If gastric cancer or other malignancy is found, the treatment regimen may need adjustment 1
• The weekly schedule bridges the gap until definitive diagnosis establishes whether she needs neurological-intensity dosing (alternate days) or standard maintenance 1

Injection Technique

• Use the deltoid or vastus lateralis muscle; avoid the buttock due to risk of sciatic nerve injury 1
• If using the gluteal region, inject only in the upper outer quadrant with the needle directed anteriorly 1
• For patients with thrombocytopenia (platelet count >50 × 10⁹/L), standard IM administration is safe 1

Monitoring During Weekly Injections

• Do not recheck B12 levels during the loading phase; wait until 3 months after starting treatment to assess response 1, 2
• Monitor for neurological symptom improvement (paresthesias, numbness, gait disturbances, cognitive difficulties, glossitis) as the primary indicator of treatment adequacy 1
• Check complete blood count at 1–2 months to confirm resolution of megaloblastic anemia 1
• Measure methylmalonic acid (MMA) and homocysteine at 3 months if symptoms persist despite treatment; target MMA <271 nmol/L and homocysteine <10 µmol/L 1, 2

Folate Co-Administration Warning

Do not give folic acid until after B12 repletion is established (typically after 4–8 weeks of weekly injections), as folic acid can mask megaloblastic anemia while allowing irreversible subacute combined degeneration of the spinal cord to progress 1, 2, 4

Transition to Maintenance Therapy

After GI evaluation determines the underlying cause:

• If pernicious anemia, ileal resection >20 cm, or malabsorption: Continue 1000 µg IM every 2–3 months for life 1, 2
• If dietary deficiency only: May transition to oral 1000–2000 µg daily 1, 4
• If neurological involvement is discovered: Switch to alternate-day dosing (1000 µg IM every other day) until neurological improvement plateaus, then every 2 months for life 1

Common Pitfall to Avoid

Do not use cyanocobalamin if this patient has renal dysfunction; hydroxocobalamin or methylcobalamin are preferred because cyanocobalamin requires renal clearance of the cyanide moiety and is associated with doubled cardiovascular event risk (HR 2.0) in patients with diabetic nephropathy 1, 2