Do anticonvulsants reduce the efficacy of oral contraceptives?

Women in our area with epilepsy who were also taking oral contraceptives were identified. Of 82 patients taking oral contraceptives, 41 had used both anticonvulsants and oral contraceptives for a total of 955 months. Three documented oral contraceptive (pill) failures occurred during this period, whereas the expected number of 0.12 (relative risk, 25; 95% confience interval, 5 to 73). No pill failures were observed in 2,278 months among women with epilepsy who were taking oral contraceptives but who were not taking anticonvulsants at this time. Thus our data support the suggestion that there is an increased rate of pill failure among women taking anticonvulsants. In view of this diminished effectiveness, the advisability of using oral contraceptives rather than one of the other forms of contraception when anticonvulsant medication is being used concurrently may need to be reevaluated.