What are the recommended vitamin B12 (cobalamin) doses for treating megaloblastic anemia according to age group?

Vitamin B12 Dosing for Megaloblastic Anemia by Age Group

For megaloblastic anemia due to vitamin B12 deficiency, administer 1000 mcg (1 mg) daily for 10 days regardless of age, followed by weekly dosing for 4 weeks, then monthly maintenance for life. 1

Initial Treatment Phase (Days 1-10)

All age groups receive the same initial dosing:

• 1000 mcg daily for 10 days (oral or intramuscular) 1
• Reticulocytosis should appear between days 5-10, confirming correct diagnosis and treatment response 1, 2
• Route selection depends on severity and neurologic involvement, not age 1

Route Selection Algorithm

Choose intramuscular administration when:

• Severe neurologic symptoms are present (peripheral neuropathy, cognitive impairment, gait disturbance) 3, 4
• Severe anemia requiring rapid correction 4
• Confirmed malabsorption (pernicious anemia, post-gastric surgery, inflammatory bowel disease) 4

Choose oral administration when:

• No severe neurologic symptoms 1
• Patient preference for ease of administration 1
• Cost is a consideration (oral is significantly less expensive) 1

The evidence demonstrates that oral and intramuscular routes are equally effective for correcting anemia and neurologic symptoms in megaloblastic anemia, with similar hematologic recovery patterns 1. A prospective randomized study of 60 patients showed identical improvements in hemoglobin, MCV, white blood cell count, platelet count, and serum B12 levels between oral and intramuscular groups at 30 and 90 days 1. Neurologic improvement occurred in 77.8% of oral patients versus 75.0% of intramuscular patients 1.

Continuation Phase (Weeks 2-4)

• 1000 mcg once weekly for 4 weeks (same route as initial treatment) 1
• Monitor hematologic parameters at day 30 to confirm response 1

Maintenance Phase (Lifelong)

Standard maintenance:

• 1000 mcg once monthly for life 1
• Alternative for pernicious anemia: Hydroxocobalamin 1 mg intramuscularly every 2 months for life 3

Special population - Post-bariatric surgery patients:

• 1000 mcg oral daily indefinitely (higher frequency due to permanent malabsorption) 3, 4

Age-Specific Considerations

Pediatric patients (childhood through adolescence):

• Use the same dosing regimen (1000 mcg daily × 10 days, then weekly × 4 weeks, then monthly) 2
• A 14-year-old vegetarian patient achieved complete clinical-hematological remission in 2 weeks with this standard protocol 2
• Reticulocyte crisis appeared within 5 days, confirming diagnosis 2

Adults age 60-74 years:

• Standard dosing applies 1
• This age group has 18.1% prevalence of metabolic B12 deficiency, making treatment particularly important 5

Adults age 75+ years:

• Standard dosing applies 1
• 25% of those ≥85 years have B12 <170 pmol/L, but dosing remains unchanged 5
• Higher risk of metabolic deficiency (18.1% in those >80 years) does not alter treatment dose 5

Critical Treatment Principles

Never administer folic acid before B12 treatment, as it may mask anemia while allowing irreversible neurological damage to progress (subacute combined degeneration of the spinal cord) 3, 5. This applies to all age groups without exception.

Seek urgent specialist consultation (neurology and hematology) if unexplained sensory, motor, or gait symptoms are present, as neurologic symptoms can become irreversible if untreated 3, 5.

Monitoring Response

• Day 5-10: Confirm reticulocytosis (proves correct diagnosis and treatment) 1, 2
• Day 30: Measure hemoglobin, MCV, WBC, platelet count (should show significant improvement) 1
• Day 90: Measure serum B12 level and complete hematologic parameters (should normalize) 1
• Neurologic assessment: Improvement typically occurs by day 30 in 75-78% of patients 1

The evidence shows no age-based dose adjustments are necessary for megaloblastic anemia treatment, as the same 1000 mcg regimen produces equivalent hematologic and neurologic recovery across all age groups from childhood through elderly populations 1, 2.