Cobalamin with a cyanide ligand at the central cobalt — must be dealkylated by cyanocobalamin reductase and reduced to cob(II)alamin before conversion to methylcobalamin (cytosol) or adenosylcobalamin (mitochondrion), the two active coenzyme forms. Oral absorption requires gastric intrinsic factor (IF) + ileal cubilin/amnionless uptake — saturable at ~1.5 mcg per dose; passive diffusion at ~1% above that.
Half-Life (t½)
PO: 144h
Dosing Guidelines
PO: Typical 400 mcg (Range: 100–1000 mcg)
SL: Typical 400 mcg (Range: 100–1000 mcg)
IM: Typical 400 mcg (Range: 100–1000 mcg)
Target Organ Systems
nervous
immune-hematologic
Interactions
Methylcobalamin (synergistic): Same family — methylcobalamin is one of the two active forms. Pick one.
Metformin (caution): Long-term metformin reduces B12 absorption. Monitor at ≥4 y use.
Adenosylcobalamin (synergistic): Same family; AdoCbl skips the cblA/cblB adenosyltransferase activation step.
Hydroxocobalamin (synergistic): Same family; OH-Cbl is the preferred parenteral form — longer t½, tighter TCII binding, no cyanide ligand.