1. We have searched the adverse reactions register for the years 1968-84 in an attempt to evaluate data relating to reported pregnancies in women on oral contraceptive steroids (OCS) who concurrently received either an antiepileptic drug or an antibiotic. 2. A total of 43 pregnancies were reported in women on OC therapy who concurrently received antiepileptic drugs and 63 pregnancies in women receiving antibiotics. In addition the number of prescriptions for both antiepileptics and antibiotics in England are reported for the years 1973-84.
PIP: To assess the extent of interactions between oral contraceptives (OCs) and antiepileptic drugs and antibiotics, the UK Committee on Safety of Medicines yellow card reporting system of adverse drug reactions was searched for the years 1968-84. A total of 43 pregnancies were reported in women on OCs who concurrently received anticonvulsant drugs and there were 63 pregnancies in OC users who used antibiotics. The most common anti-epileptic drugs involved in reported adverse interactions with OCs were phenytoin (25 cases) and phenobarbitone (20 cases); the antibiotic most often implicated in adverse OC-drug interactions was penicillin (32 cases). The highest number of reports occurred in 1973 for anti-epileptics (6 cases) and in 1982 for antibiotics (14 cases). The largest number of reports included OCs containing ethinyl estradiol (30 mcg) and levonorgestrel (0.15 mg). Since only about 15% of physicians in the UK are believed to use the yellow card scheme for reporting adverse drug reactions, these data are not indicative of the actual prevalence of such reactions. However, they do indicate that contraceptive failure may occur in some women exposed to these 2 groups of commonly prescribed drugs. There is research evidence that broad spectrum antibiotics alter plasma concentrations of OC steroids. Caution should be used in generalizing these findings, however, until a clearer picture emerges as to which OC users are most susceptible to adverse drug interaction effects.