PIP: This is a brief review of the theoretical and known drug reactions with oral contraceptives. There are at least 6 possible types of drug reactions that may affect the action of oral contraceptives, not including malabsorption related to changes in intestinal motility or flora. Ampicillin is an example of an antibiotic that may cause diarrhea, thereby reducing absorption of pill steroids. The steroids in orals are subject to enterohepatic circulation, which is in turn affected by the gut flora. Antibiotics known to suppress gut flora include: penicillins, cephalosporins, tetracyclines, sulfas, neomycin and erythromycin. Although controlled clinical trials of antibiotic intake with oral contraception have not shown significant interactions, anecdotal reports of pill failures have been published. The other important drug interaction affecting contraception by orals is enhanced hepatic degradation, as seen with rifampicin. Other drugs such as cimetidine, MAO-inhibitor antidepressants, chloramphenicol, influenza or BCG vaccine, isoniazid, warfarin, metronidazole and disulfiram may delay steroid metabolism and possibly increase side effects. When prescribing drugs it is important to realize that certain drugs decrease oral contraceptive concentrations: antibiotics anticonvulsants, griseofulvin, purgatives and rifampicin.