PIP: As drug interaction can play a part in altering biological effectiveness of the administered agents, effective menstrual control by oral contraceptives may be jeopardized by simultaneous therapy with other drugs. And, conversely, oral contraceptives may alter the biotransformation of other therapeutic agents. The clinical literature concerning with drug induced contraceptive failure is still limited and apart from 1 or 2 exceptions consists of case reports generally on few patients. In a well-controlled study, Hempel and Klinger administered different psychotropic drugs to women taking 0.05 mg ethinyloestradiol plus 1.0 mg norethisterone acetate without menstrual dysfunction. Phenobarbitone and carbamazepine most frequently provoked bleeding disorders. It has been suggested that various anti-epileptic drugs will reduce the effectiveness of oral contraceptive steroids. Phenytoin administered experimentally to 14 women on oral contraceptives provoked bleeding disorders in 3. Janz and Schmidt refer to 3 patients becoming pregnant while on anti-epileptic drugs and oral contraceptives. Primidone, phenobarbitone and ethosuccimide were implicated. Gagnaire et al. and Belaish et al. cite individual cases of conception in patients treated with anti-epileptics including primidone given alone. Rifampicin is the antibiotic most clearly implicated in menstrual disorders and conception in women taking sequential or combined oral contraceptives. The main site of interaction of oral contraceptives with other drugs is in the liver where they share the same metabolising enzymes.