Phenytoin

Phenytoin, usually sold under the brand name Dilantin(TM) is a medication used primarily to treat tonic-clonic and partial seizures, although it is also used to treat certain types of arrhythmia and pain caused by neuropathies. It can be taken orally or administered intravenously. It's use in seizures was first discovered in 1936 and, because it is off-patent, it is still in use as a monthly supply can usually be obtained for about $30 U.S.
It has numerous side effects including nausea, stomach pain, and anorexia. Less common but more serious side effects include sleepiness, self-harm, liver damage, bone marrow suppression, low blood pressure and toxic epidermal necrolysis. It is contraindicated in pregnant women as it can cause birth defects, but it is generally safe for nursing mothers.
Phenytoin works by blocking sodium channels that are voltage dependent, It binds to inactive sodium channels making them harder to activate. 
Phenytoin at Wikipedia