Interactions Between Antibiotics and Oral Contraceptives

To the Editor.— The response given in the QUESTIONS AND ANSWERS section to an inquiry concerning concurrent oral contraceptive (OC) and antibiotic therapy (1981;246:2076) warrants some comments. Dr Proudfit states that reported interactions of rifampin, ampicillin, and tetracycline with OCs presumably result because these drugs induce (I must assume the word include is a misprint in the reply, and the intended word is induce) hepatic microsomal enzymes. While this is certainly important in the case of rifampin, ampicillin and tetracycline are not known to induce hepatic enzymes. Rather, the latter two drugs probably interact with OCs by another, unrelated mechanism. Synthetic estrogens undergo extensive enterohepatic recirculation, a process that probably involves the gut flora. The glucuronidated metabolites of the estrogens are secreted into the bile but cannot be reabsorbed from the intestine until the glucuronide moiety is hydrolyzed to regenerate free drug, a reaction that is carried out to a