Treatment of Women With Epilepsy

Drug Interactions

Oral hormonal contraceptive medications taken in combination with hepatic cytochrome P450 enzyme-inducing AEDs can be ineffective ( Table 1 ). A 6% per year failure rate of oral contraceptives in women taking hepatic enzyme-inducing AEDs has been reported.^[23,24,25]

Oral contraceptive failure is due to the increased metabolism of sex steroid hormones by the cytochrome P450-inducing AEDs.^[26] Standard prescribed oral contraceptives contain only the minimal dosage of hormone required to inhibit ovulation. Even small increases in metabolism may lead to contraceptive failure. In order to provide acceptable contraceptive efficacy, women taking cytochrome P450-inducing AEDs must receive at least 50 µg of the estrogen component.^[27,28] Long-term progesterone-only contraceptive systems, such as subdermal levonorgestrel, are also prone to failure because of increased steroid metabolism.^[29,30] Intramuscular medroxyprogesterone (Depo-Provera) has not been evaluated for efficacy in women receiving cytochrome P450-inducing AEDs. For women receiving hepatic enzyme-inducing AEDs in whom pregnancy is contraindicated, use of a barrier contraceptive should be advised.

Unfortunately, most physicians are not aware of this interaction, as demonstrated by two surveys. One was a United States-based national survey of neurologists and obstetricians,^[31] and the other surveyed more than 3500 health care providers including pediatricians, general practitioners, and family practice physicians.^[32]

Semin Neurol. 2002;22(3) © 2002  Thieme Medical Publishers