Recommended Vitamin B12 Dosage for Adult Deficiency
For adults with vitamin B12 deficiency, the recommended dosage is 1000-2000 μg (1-2 mg) of cyanocobalamin daily orally or 1000 μg intramuscularly monthly. 1
Treatment Options Based on Deficiency Cause
Oral Supplementation
- Oral dosage: 1000-2000 μg daily 1
- FDA-approved oral dosing: 1000 μg (1 tablet) daily, preferably with a meal 2
- Oral route is as effective as intramuscular administration for most patients 1
- Sublingual formulations can also be used at the same dosage
Intramuscular (IM) Supplementation
- IM dosage: 1000 μg monthly 1
- Loading dose regimen: 1000 μg IM given 5-6 times biweekly 3
- Maintenance: 1000 μg IM monthly 1, 3
- Preferred for patients with severe malabsorption 4
Special Population Considerations
Malabsorption Disorders
- Post-bariatric surgery: 1000 μg oral B12 daily indefinitely 1
- Crohn's disease with ileal involvement/resection: 1000 μg monthly if >20 cm of distal ileum is resected 1
- Oral cyanocobalamin has been shown effective in 94.7% of Crohn's patients with B12 deficiency 5
Dietary Deficiency
- Vegans and vegetarians: 250-350 μg daily or 1000 μg weekly 1
- Breastfeeding mothers: At least 2.8 mg cyanocobalamin per day orally 1, 6
Important Clinical Considerations
Duration of Treatment
- Pernicious anemia: Lifelong monthly injections required 6
- Other causes: Treatment duration depends on underlying cause but typically continues long-term
Monitoring
- Check vitamin B12 levels after 3 months of supplementation 1
- Measure MMA levels if B12 levels remain indeterminate (180-350 ng/L) 1
- Monitor for other nutrient deficiencies, particularly folate 1
Safety Considerations
- Vitamin B12 has excellent safety profile even at high doses 1
- No established upper tolerable limit defined 1
- Elevated B12 levels due to supplementation are not associated with toxicity 1
- Vitamin B12 deficiency left untreated for >3 months may produce permanent degenerative lesions of the spinal cord 6
Dose Efficacy
- Research shows that doses of 647-1032 μg daily are needed to achieve 80-90% of maximum reduction in plasma methylmalonic acid 7
- The lowest effective dose for normalizing mild B12 deficiency is approximately 200 times greater than the recommended dietary allowance of 3 μg daily 7
Treatment Algorithm
Confirm deficiency using appropriate testing:
- Total B12 <180 ng/L or active B12 <25 pmol/L: Confirmed deficiency
- Indeterminate results: Measure serum MMA
Determine cause of deficiency:
- Malabsorption (pernicious anemia, gastric/ileal surgery, Crohn's disease): Consider IM route
- Dietary insufficiency: Oral supplementation adequate
Initiate treatment:
- Oral: 1000-2000 μg daily
- IM: Loading with 5-6 biweekly injections of 1000 μg, then monthly maintenance
Monitor response after 3 months
- If symptoms persist or B12 levels remain low: Consider switching from oral to IM route or increasing frequency of IM injections
Maintain treatment based on underlying cause:
- Irreversible causes (pernicious anemia, surgical resection): Lifelong therapy
- Reversible causes: Continue until cause is addressed and normal levels maintained
Remember that early treatment is essential to avoid irreversible neurological consequences, and individualized injection regimens may be necessary for some patients to remain symptom-free 4.