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.

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 hospitals 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.

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