Fe: Inhibits absorption of tetracyclines. May delay response to Fe therapy w/ chloramphenicol. May decrease absorption w/ antacids & cholestyramine. May interfere w/ the oral absorption of some quinolones (eg, ciprofloxacin, norfloxacin, ofloxacin). Decreased cupruretic effect of penicillamines. Inhibited absorption w/ eggs, milk, phytate-containing substances eg, bran or tannins in tea. Hinder detection of occult blood in feces. False +ve for guaiac test. Ascorbic acid: Decreased prothrombin time w/ anticoagulants. Interfere w/ tests based on oxidation-reduction reactions. May produce false results in glucosuria determinations in the urine. Niacinamide: Toxic delirium & lactic acidosis w/ alcohol. Increased flushing & dizziness w/ nicotine. May potentiate the hypotensive effect of ganglionic blocking drugs. May enhance the effects of neuromuscular blocking agents. May cause false elevations in fluorometric tests of urinary catecholamines. False +ve results when cupric sulfate soln (eg, Benedict's reagent) is used for urinary glucose determination. May change dosage requirements of anti-diabetic agents. Thiamine: False +ve results in the phosphotungstate method for determination of uric acid. False +ve results for urobilinogen in the spot test using Ehrlich's reagent. Riboflavin: May delay absorption time but increase the absorption conc w/ propantheline Br. May interfere w/ urinalysis based on spectrometry or color reactions. False elevations in fluorometric determinations of catecholamines or urobilinogen. Pyridoxine HCl: Decreased serum conc of phenobarb or phenytoin. False +ve results for urobilinogen in the spot test using Ehrlich's reagent. Possible interaction w/ levodopa. Cyanocobalamin: May diminish response w/ chloramphenicol. Diminished absorption w/ aminoglycosides, colchicine, ER K prep, aminosalicylic acid & its salts, anticonvulsants, cobalt irradiation of the small bowel or chronic excessive alcohol intake. Invalidate diagnostic microbiologic blood assays w/ MTX, pyrimethamine & most anti-infectives. False +ve test results for IF Abs.