Patient Encounter 4 Hormonal Contraceptives and Interactions With AEDs

LJ, a 25-year-old-woman with complex partial seizures, presents a prescription to the pharmacy for a triphasic oral contraceptive containing ethinyl estradiol and norges-timate. A review of her medication profile shows that she is taking carbamazepine extended release 1,200 mg/day. Her last refill for this prescription was 2 weeks ago. She reports that she has not had a seizure for 1 year and that she just became engaged. She is planning to be married in 4 months.

Is there an interaction between carbamazepine and the oral contraceptive? If so, what is the cause and clinical outcome of the interaction? If there is an interaction, how should it be managed? What AEDs interact with hormonal contraceptives?

What are the clinical implications of these interactions? How should they be managed?

As noted above, many of the AEDs induce hepatic microsomal enzyme systems and thus reduce the effectiveness of hormonal contraceptives. Women taking AEDs that may reduce the effectiveness of hormonal contraceptives should be encouraged to also use other forms of birth control. In contrast to these interactions, hormonal contraceptives induce glucoronidation of lamotrigine and valproate. Oral contraceptives that cycle hormones cause reductions in serum concentrations of lamotrigine or valproate during days of the cycle when hormones are taken; serum concentrations increase during days when hormones are not taken. Due to induction or inhibition of sex hormone metabolism and changes in binding of hormones to sex hormone binding globulin, some AEDs may reduce fertility. For example, valproate has been associated with a drug-induced polycystic ovarian syndrome. Women who experience difficulties with fertility should seek the advice of health care professionals with expertise in fertility.

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