Oral contraceptive drug interactions. 
Approximately 50 million women use oral contraceptives (OC).  Studies and case reports demonstrate that OC failure may be caused by rifampin, anticonvulsant drugs, and possibly some antibiotics.  Contraceptive steroids may interfere with the metabolism of the benzodiazepines, theophylline, and the glucocorticoids.  Future investigation will document the clinical significance of other OC interactions as well as give rise to new interactions.