Cyanocobalamin
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Cyanocobalamin (vitamin B12)
Many OTC and Rx; Nascobal (intranasal); generic injection
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Summary
Common uses
Vitamin B12 deficiency0, Pernicious anemia0, Post-bariatric / malabsorptive supplementation0, Vegetarian/vegan supplementation0
Pharmacy
Starting dose
Replacement: 1000 mcg IM daily for 1 week, then weekly for 4 weeks, then monthly; or 1000-2000 mcg PO daily (effective even in pernicious anemia via passive diffusion); intranasal 500 mcg weekly
Preparations
100, 250, 500, 1000, 5000 mcg tablets (OTC and Rx); 1000 mcg/mL injection; intranasal spray; sublingual
US FDA Max
No strict ceiling; water-soluble vitamin, low toxicity
Pharmacology
Routes
Oral, sublingual, intranasal, IM, SC, IV
Onset
Reticulocyte response at 3-5 days; neurologic recovery weeks to months (and may be incomplete if longstanding)
Duration
N/A (replacement)
Half-life
~6 days (plasma); hepatic stores last 3-5 years
Bioavailability
Oral ~1-3% via passive diffusion at high doses (independent of intrinsic factor); IM/SC ~100%
Pregnancy
Routinely supplemented in vegan pregnancies and pernicious anemia.[citation needed]
Legal status
OTC (low/mid-dose oral) and Rx-only (injection, intranasal) in US
Purported mechanism
Cyanocobalamin is an inactive storage/synthetic form of vitamin B12 that is converted intracellularly to methylcobalamin (cofactor for methionine synthase, regenerating methionine and tetrahydrofolate) and adenosylcobalamin (cofactor for methylmalonyl-CoA mutase, fueling propionyl-CoA / TCA entry).0 Failure of either coenzyme produces the characteristic megaloblastic anemia (THF trap) and subacute combined degeneration of dorsal columns and corticospinal tracts. Pernicious anemia is treated with high-dose oral or parenteral B12 because the autoimmune intrinsic factor loss is bypassed by mass-action passive diffusion at supraphysiologic doses.