Phenytoin used to be first line treatment for epilepsy not too long ago but has been relegated to third line therapy in favour of newer drugs.
The reason phenytoin was relegated was that the newer drugs caused fewer side effects and do not have the same complex pharmacokinetics as phenytoin. Plasma concentrations of phenytoin is directly proportional to the oral dose of phenytoin given ... up to a point. At this point, the enzyme (P450, I think) involved in metabolising phenytoin is saturated and a small increase in the oral dose can result in a large increase in plasma levels of phenytoin.
Phenytoin is still used in the emergency setting for status epilepticus as it is still readily available in intravenous form.