Recommended Oral Vitamin B-12 Supplementation Dose
For adults at risk of vitamin B-12 deficiency, take 1000-2000 mcg (1-2 mg) of oral cyanocobalamin daily, which is as effective as intramuscular administration for most patients and corrects both anemia and neurologic symptoms. 1, 2, 3
Standard Dosing by Clinical Scenario
General Prevention and Maintenance
- Adults over 50 years: 500-1000 mcg daily of crystalline vitamin B-12 to overcome age-related absorption decline from atrophic gastritis 1, 4, 5
- Vegans/strict vegetarians: 1000 mcg daily or consume fortified foods 2, 3
- Metformin users (>4 months): 1000-2000 mcg daily due to medication-induced malabsorption 1, 2, 3
- PPI or H2 blocker users (>12 months): 1000-2000 mcg daily 1, 3
Confirmed Deficiency Treatment
- Standard deficiency (serum B-12 <180 pg/mL): 1000-2000 mcg daily until levels normalize, then continue as maintenance 1, 2, 3
- Post-bariatric surgery patients: 1000-2000 mcg daily orally indefinitely, or 1000 mcg intramuscular monthly for life 6, 1, 7, 2
- Ileal resection >20 cm: 1000 mcg intramuscular monthly for life (oral may be insufficient) 1
When to Switch to Intramuscular Therapy
Use intramuscular hydroxocobalamin 1000 mcg on alternate days until no further neurological improvement, then 1000 mcg every 2 months for life if: 1, 8, 3
- Severe neurological symptoms present (peripheral neuropathy, ataxia, cognitive impairment)
- Oral therapy fails to normalize levels after 3 months
- Confirmed pernicious anemia or intrinsic factor deficiency
- Severe malabsorption documented
Critical Dosing Context
The recommended dose of 1000-2000 mcg daily is more than 400 times the recommended dietary allowance of 2.4 mcg/day because absorption is severely impaired in at-risk populations—only 1-2% of oral B-12 is absorbed passively without intrinsic factor. 1 This high dose ensures adequate absorption through passive diffusion mechanisms that bypass the need for intrinsic factor. 2, 5
Monitoring Schedule
- Recheck serum B-12 at: 3 months, 6 months, and 12 months in the first year 1, 7
- Annual monitoring once levels stabilize 1
- Measure at follow-up: serum B-12, complete blood count, methylmalonic acid (MMA), and homocysteine to assess treatment response 1
Safety and Form Selection
Vitamin B-12 has no established upper toxicity limit—excess amounts are readily excreted in urine without harm, making high-dose supplementation extremely safe for long-term use. 1, 7, 4
Preferred forms: 1
- Cyanocobalamin: Standard form for oral supplementation (FDA-approved at 1000 mcg daily) 9
- Methylcobalamin or hydroxocobalamin: Preferred over cyanocobalamin in patients with renal dysfunction, as cyanocobalamin requires renal clearance of the cyanide moiety and is associated with increased cardiovascular events 1
Critical Pitfall to Avoid
Never administer folic acid before ensuring adequate B-12 treatment, as folic acid can mask the anemia of B-12 deficiency while allowing irreversible neurological damage to progress. 1, 7, 2 Always check and treat B-12 deficiency first, then address folate if needed.
When Standard Dosing May Be Insufficient
Up to 50% of individuals with B-12 malabsorption require individualized regimens with more frequent intramuscular administration (ranging from twice weekly to every 2-4 weeks) to remain symptom-free, as responses to treatment vary considerably. 8 If symptoms persist despite 3 months of oral therapy at 1000-2000 mcg daily, switch to intramuscular therapy rather than increasing oral dose. 1, 8