Interactions between antiepileptic drugs and hormonal contraception

An interaction between antiepileptic drugs (AEDs) and the combined oral contraceptive pill was first proposed when the dose of estradiol in the oral contraceptive pill was reduced from 100 to 50 microg. There was a higher incidence of breakthrough bleeding and contraceptive failure among women with epilepsy compared with women in general. Since then, interaction studies have been undertaken to look for possible interactions between AEDs and the combined oral contraceptive pill. Phenobarbital (phenobarbitone), phenytoin, carbamazepine, oxcarbazepine, felbamate and topiramate have been shown to increase the metabolism of ethinylestradiol and progestogens. Therefore, if a women is on one of the AEDs and wishes to take the oral contraceptive pill, she will need to take a preparation containing at least 50 microg of ethinylestradiol. Levonorgestrel implants are contraindicated in women receiving these AEDs because of cases of contraceptive failure. It is recommended that medroxyprogesterone injections be given every 10 rather than 12 weeks to women who are receiving AEDs that induce hepatic microsomal enzymes. There are no interactions between the combined oral contraceptive pill, progesterone-only pill, medroxyprogesterone injections or levonorgestrel implants and the AEDs valproic acid (sodium valproate), vigabatrin, lamotrigine, gabapentin, tiagabine, levetiracetam, zonisamide, ethosuximide and the benzodiazepines. Therefore, normal dose contraceptive preparations can be used in patients receiving these AEDs.