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Antiepileptics and Oral Contraceptives: A Reminder

As the number of available drugs for psychiatric conditions increases, so does the likelihood of drug interactions. This study randomly surveyed 1000 neurologists and 1000 obstetricians nationwide and found that few fully know about antiepileptic drugs' effects on oral contraceptive (OC) sex-hormone levels or the potential for antiepileptics to cause birth defects.

Most of the 307 respondents said they treated epileptic women of childbearing age. Although 75% of neurologists and 59% of obstetricians claimed to be familiar with antiepileptic-OC interactions, only 4% of neurologists and none of the obstetricians correctly stated which of six common antiepileptic drugs interfere with OCs. Forty-four percent of neurologists and 23% of obstetricians underestimated the risk of birth defects with antiepileptics as 0-3% (the true risk is 4%-6%). Only a minority of both groups reported increasing the OC doses for patients on antiepileptics to prevent accidental pregnancy.

Comment: Although survey response rates were only about 15%, the results are very important clinically. The use of antiepileptics has become more common in psychiatry, and many of our patients are in their childbearing years. It is crucial that we become aware of these drug interactions and the risk of birth defects from the use of antiepileptics. The oral contraceptives are metabolized by P450 IIIA4; carbamazepine, phenytoin, phenobarbital, primidone, and ethosuximide will reduce OC estradiol levels by 40% through their effect on the P450 system, and they may also reduce free progestin levels. Valproic acid and gabapentin do not induce the P450 system, and do not affect OC levels.

— G Tucker

Published in Journal Watch Psychiatry August 1, 1996

Citation(s):

Krauss GL et al. Antiepileptic medications and oral contraceptive interactions: a national survey of neurologists and obstetricians. Neurology 1996 Jun 46 1534-1539.

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