There are complex interactions between the hormones (estrogen and progesterone) contained in birth control pills or devices, and some of the medications used to control seizures. Some of these medications increase the breakdown of contraceptive hormones in the body, making them less effective in preventing pregnancy. The seizure medications that have this effect are often called "liver enzyme-inducing" drugs because the liver is the organ that breaks down these hormones. They are carbamazepine (Tegretol, Carbatrol), oxcarbazepine (Trileptal), phenytoin (Dilantin), phenobarbital (Luminal), primidone (Mysoline), and topiramate (Topamax). Valproate (Depakote) and felbamate (Felbatol) do not increase breakdown of hormones, and may even increase hormonal levels, which may require an adjustment in the dose of your birth control. Gabapentin (Neurontin), lamotrigine (Lamictal), levetiracetam (Keppra), and tiagabine (Gabitril) have no effect on this system and do not interfere with the effectiveness of hormonal birth control.
The popular low-dose combined oral contraceptive pill has a relatively small amount of estrogen (less than 35 micrograms). That's not enough to protect women with epilepsy who take enzyme-inducing AEDs from becoming pregnant. You may need contraceptive pills with higher doses of estrogen, and even then, there is a risk of unexpected pregnancy. It is a good idea to use barrier methods (a diaphragm, spermicidal cream or a condom) in addition to the contraceptive pill, if you are taking one of the seizure medications that speed up the breakdown of the hormones in birth control pills.