Interactions between oral contraceptives and other drugs

: Failures of oral contraceptives are possible when combined with rifampicin or antiepileptics, especially phenobarbitone and phenytoin. The mode of action is shown by clinical trials to be due to induction of hepatic enzymes thus increasing the steroid metabolism. Failure or oral contraceptives has occurred with the concomitant use of antibiotics, i.e. ampicillin and sulfonamides. Clinical trials have focused upon the changes in the intestinal flora induced by antibiotics. This might influence the enterohepatic circulation of estrogen and thereby decrease reabsorption of estrogen, but this has not been definitely proved. The failures may be caused by individual pharmacokinetics of oral contraceptives. Oral contraceptives are able to influence the pharmacodynamics of various other drugs metabolized by oxidation or conjugation but besides an increased pharmacological effect of prednisolone and increased toxicity of imipramine the clinical importance is uncertain.