Pyridoxine: Reduced effects of levodopa. Reduced activity of altretamine. Decreased serum conc of phenobarb & phenytoin. Requirements for pyridoxine may be increased w/ hydralazine, INH, penicillamine, & OCs. Cyanocobalamin: Absorption from GI tract may be reduced w/ neomycin, ASA, histamine H
2-antagonists, omeprazole. & colchicine. Serum conc may be decreased w/ OCs. Vit B
12 anemia may be attenuated w/ parenteral chloramphenicol.