1) Many of the medicines used to treat epilepsy and
bipolar disorder—Barbituates, Carbamazepine, Oxycarbazepine,
Phenytoin, Primidone, Topiramate, Felbamate, and Lamotrigine when taken alone—make the pill, patch, ring and mini-pill less effective, increasing the chance of an accidental pregnancy.
2) There’s some evidence that these methods of birth control change the effectiveness of those particular medicines, too.