Phenytoin, the epilepsy medicine, can make Levonorgestrel [For emergency contraception] work less well, or even stop it working. The same is true for other seizure medicines such as phenobarbitone, primidone and carbamazepine. Rifampicin and griseofulvin can also weaken the effect of Levonorgestrel [For emergency contraception].
These medicines are often taken together with some care. Your doctor may change a dose, space them apart or check on you more often. Make sure your doctor and pharmacist know you take both.