Description
Vitamins B12 is irregularly absorbed from the distal small intestine following oral
administration. Vitamin B12 absorption is an active process that requires gastric intrinsic factor.
Intrinsic factor is a glycoprotein secreted by the gastric mucosa. Passive diffusion through the
intestinal wall can occur but large amounts of B12 are required (i.e. >1 mg). Following oral doses
less than 3 mcg, peak plasma concentrations are not reached for 8 to 12 hours because the
vitamin is transiently retained in the wall of the lower ileum.
Vitamin B12 is rapidly absorbed from intramuscular (IM) and subcutaneous (SC) sites of
injection; peak plasma concentrations are reached within 1 hour after IM injection.
