The effect of thiamine is neutralized by antagonists (thiosemicarbazone) and 5-fluorouracil. Patients using any of these treatments may need their thiamine dose adjusted. Pyridoxine increases metabolism of barbiturates and phenytoin. Chloramphenicol and colchicine decreases serum concentration of Cyanocobalamin. Avoid intake of drugs with same content to prevent overdose. Heavy alcohol intake for more than two weeks causes malabsorption of Vitamin B12. Levodopa action is antagonized by Vitamin B6.