Interferences between drugs and oral contraceptives are considered to alter pharmacokinetics and thus the efficacy of steroidal hormones. It should be noted, however, that steroids can also modify the metabolism and pharmakodynamic effects of various substances. To the present knowledge, phase I (i.e., oxidation, demethylation) and phase II reactions (conjugation) are concerned. Drugs sharing those enzymatic systems with oral contraceptives experience either an increase in bioavailability by inhibition of oxidative metabolism or undergo accelerated elimination by induced conjugation. Such interaction may be of practical interest in subjects who take oral contraceptives and are simultaneously treated with antidepressants, anti hypertensives, insulin, synthetic glucocorticoids, theophylline, and caffeine.