Mecobalamin and Cyanocobalamin Are Therapeutically Equivalent on a Microgram-for-Microgram Basis
5000 mcg of mecobalamin (methylcobalamin) converts to 5000 mcg of oral cyanocobalamin—there is no dose adjustment needed between these two forms of vitamin B12.
Direct Conversion Principle
- Both mecobalamin (methylcobalamin) and cyanocobalamin are forms of vitamin B12 that are converted in the body to the two active coenzyme forms: methylcobalamin and adenosylcobalamin 1
- Cyanocobalamin is converted to both active forms (methylcobalamin and adenosylcobalamin), while supplemental methylcobalamin provides only one of the two essential active forms 1
- The bioavailability and therapeutic efficacy are equivalent when comparing equal microgram doses of oral cyanocobalamin and methylcobalamin 1
Bioavailability Context
- At low physiologic doses approximating daily requirements (around 2.3 mcg), oral cyanocobalamin has a bioavailability of approximately 46% 2
- At higher therapeutic doses (18.3 mcg and above), bioavailability decreases to approximately 7-8% due to saturation of intrinsic factor-mediated absorption 2
- However, at doses of 1000-2000 mcg daily, passive diffusion becomes the dominant absorption mechanism, making oral cyanocobalamin therapeutically equivalent to intramuscular administration even in patients with malabsorption 3, 4
Clinical Application for 5000 mcg Dosing
- For therapeutic purposes, 5000 mcg of mecobalamin should be replaced with 5000 mcg of oral cyanocobalamin taken daily 5, 3
- This high dose ensures adequate absorption through passive diffusion (approximately 1% of the dose), providing 50 mcg of absorbed vitamin B12, which far exceeds daily requirements 4
- Standard therapeutic dosing for B12 deficiency ranges from 1000-2000 mcg daily orally, so 5000 mcg represents a higher-than-typical but safe therapeutic dose 5, 3
Important Considerations
- Cyanocobalamin is actually preferred over methylcobalamin alone because it provides both active coenzyme forms (methylcobalamin and adenosylcobalamin) after conversion in the body 1
- Methylcobalamin supplementation only provides one active form and may not adequately address adenosylcobalamin-dependent metabolic pathways involved in myelin formation 1
- There is no established upper toxicity limit for cobalamin, and no reports of acute toxicity with oral supplementation at any dose 5
- The oral route has been proven effective even in conditions with impaired absorption, including Crohn's disease with ileal resection, when using doses of 1000-1200 mcg daily 6, 3
Practical Dosing Algorithm
- If converting from 5000 mcg mecobalamin to cyanocobalamin: prescribe 5000 mcg oral cyanocobalamin daily 5
- Alternatively, if the goal is standard therapeutic dosing for deficiency treatment, 1000-2000 mcg oral cyanocobalamin daily is sufficient and evidence-based 5, 3
- Monitor treatment adequacy at least annually with resolution of clinical symptoms and laboratory markers (serum B12, methylmalonic acid, homocysteine) 7, 5