Phenytoin works in epilepsy by promoting sodium efflux from the motor cortex neurons and thus slows conduction within the motor cortex.
Unfortunately it is an enzyme inducer (increases the metabolism of hepatically metabolised drugs) so has multiple drug interactions and side effects:
- Drowsiness; fatigue; ataxia; irritability; headache; restlessness; slurred speech;
- Nystagmus and ophthalmoparesis
- megaloblastic anaemia (decreases folate levels)
- Gingival enlargement
Phenytoin toxicity is not uncommon when taken regularly so it is not commonly prescribed first line as a long term, prophylactic antiepileptic. It can, however, be given safely in high load so can be acutely given in seizure management.