Interactions with antiepileptic drugs

Most of the currently available antiepileptic drugs have a low therapeutic ratio and therefore a drug interaction causing elevation of the serum level of one of these compounds can readily lead to drug intoxication. Phenytoin, in particular, is vulnerable because its metabolism is dose-related and at therapeutic serum levels the enzyme system involved in its degradation is easily inhibited by concurrent drug administration. As multiple drug therapy has traditionally been practised in the management of epilepsy, clinically important interactions are common. Furthermore, most of the drugs used in the treatment of major epilepsies are potent inducers of hepatic microsomal enzymes and can therefore stimulate the metabolism of concurrently-administered drugs to such an extent that they may be rendered ineffective. The use of one drug alone is recommended, where possible, in the management of epilepsy.