Hi,
AED's (Anti-epileptics drugs) increase the metabolism and elimination of estrogen from the body. The effects of increased metabolism of estrogen and decreased availability of progestin in the body decreases the effectiveness of oral contraceptives in preventing pregnancy in some women. Therefore, your physician may need to provide an oral contraceptive which contains a higher amount of estrogen than you might otherwise need to protect you against pregnancy. (Dose to titrated by your Ob/GY)
The other option is to try a newer antiepileptic like LAMICTAL that has no interaction with birth control pills (To be decided by your neurologist)
For more details try these links (they talk more about pregnancy but the basic mechanism is the same):
http://www.epilepsyfoundation.org/Forums/messageview.cfm?catid=4&threadid=11560
http://w3.ouhsc.edu/neuro/division/cope/bcp.htm
Do let me know if you need more information, please remember to 'accept' the reply if you find it useful; a positive rating & a bonus at your discretion would be appreciated,
Regards
Dr. Gupta