Absorption of Vitamin B12 from the GIT may be reduced by Neomycin, Amino salicylic acid, Histamine H2 antagonist, Omeprazole and Colchicines. Serum concentration may be decreased by use of oral contraceptives. Many of this interaction are unlikely to be of clinical significance but should be taken into account when performing assays for blood concentration. Parenteral chloramphenicol may attenuate the effect of Vitamin B12 in anemia.