The action and effectiveness of most estroprogestational contraceptive agents can be impaired by interference with administration of certain antibiotics which can modify the hepatic estrogen metabolism. Such antibiotics are: 1) rifampicin, which can inactivate the action of ethinyl estradiol; spotting can be present; 2) ampicillin, chloramphenicol, neomycin, nitrofurantoin, sulfamethoxypyridazine, and penicillin; these agents modify the intestinal bacterial flora, thus reducing the effectiveness of contraception; and 3) troleandomycin, which increases hepatic risk.