Also called diphenylhydantoin, phenytoin is a drug
that is useful in treating tonic clonic seizures
. It reduces neuron
excitability by slowing the rate of recovery of sodium channels. This drug
is not the first choice for control of epilepsy
, that would perhaps be carbamazepine
, but it is used in hospital
s for controlling many kinds of seizures, and in epilepsy
where other drugs have been unsuccessful.
Phenytoin exhibits a property called zero-order kinetics. This is because the enzymes in the liver that metabolise phenytoin easily get close to saturation. Then, a small additional dose of the drug will cause a large increase in its concentration in the bloodstream. It should be used cautiously in people with hepatic impairment (reduced liver function). Alcohol is another drug that displays this property.
Side effects of phenytoin that are not related to dose include skin rashes, gingival hyperplasia (gum overgrowth), and hirsutism (hairiness). Dose related side effects include nausea and vomiting, confusion, dizziness, headache and insomnia. Overdose causes ataxia (incoordination), slurred speech, nystagmus (eye wiggles) and blurred vision. Unfortunately, signs of toxicity can mimic the symptoms of epilepsy.
It can be taken orally, or by intravenous injection. Blood samples are taken to measure the amount of drug present in order to adjust dosage. The optimal plasma concentration for phenytoin is 10 - 20 mg/litre (40 - 80 micromol/litre). However, patients should be considered individually. Some may have good seizure control at plasma levels above or below this.
There are many drug interactions with phenytoin. It is an enzyme inducer, which means that it stimulates the liver enzymes to break down drugs faster. This can make other drugs more effective or less effective. Other drugs can have a tendency to increase the plasma concentration of phenytoin. Care must be taken when considering any other drug in combination with this one.