Delayed or interrupted reticulocyte response to supplemental vit B
12 w/
chloramphenicol. Reduced absorption of vit B
12 w/ aminosalicylic acid, antibiotics, colchicine, cholestyramine, H
2 blockers, metformin, neomycin, nitrous oxide, OCs, phenytoin, phenobarb, primidone, PPI, zidovudine.