Reduced absorption of vit B
12 from the GIT w/ neomycin, aminosalicylic acid, histamine H
2-antagonists, omeprazole & colchicine. Serum conc may be decreased w/ OCs. Parenteral chloramphenicol may attenuate the effect of vit B
12 in anemia. May experience additive CNS side effects w/ opiates or benzodiazepines. May potentiate impairment of motor skills & sedating effects of alcohol. Methylcobalamin: May alter intestinal microflora & decrease the absorption of w/ antibiotics; metformin, para-aminosalicyclic acid & KCl. May decrease enterohepatic re-absorption w/ cholestyramine, colchicine or colestipol. Produce functional deficiency w/ nitrous oxide.