May impair absorption by formation of insoluble complexes w/ compd containing Ca & Mg including antacids & mineral supplements & bicarbonates, carbonates, oxalates or phosphates. Diminished absorption of both Fe salts & tetracyclines if taken concomitantly. May decrease absorption w/ Zn salts. May increase absorption w/ ascorbic acid & citric acid. Delayed response to Fe w/ systemic chloramphenicol. Decrease absorption, bioavailability & clinical effect of biphosphates, entacapone, fluoroquinolones, levodopa, methyldopa, penicillamine & tetracycline. May reduce efficacy of thyroxin (levothyroxine).