About Phenytoin / Dilantin / Eptoin Lab Test

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The drug in question in our case was phenytoin which is available in the market in two different strengths: Eptoin (each 5 mL containing 30 mg phenytoin) and Dilantin (each 5 mL containing 125 mg phenytoin). Phenytoin is an antiepileptic drug widely used for treating several types of seizures. It is typically administered orally as phenytoin itself or else by intravenous infusion (IV) or intramuscular injection (IM) of the prodrug fosphenytoin, a relatively water-soluble phosphate ester of phenytoin. Depending on the route of administration, conversion of fosphenytoin to phenytoin is expected to be essentially complete within 2 - 4 hrs. Most of the Phenytoin in circulation is tightly bound to plasma proteins, chiefly albumin. Normally only about 10% circulates in the pharmacologically active free form, though in some conditions, such as renal failure, this fraction may increase to 30% or more. The amount circulating in free form is expected to be increased in hypoalbuminemia and in conditions where other compounds compete with phenytoin.


Phenytoin is an antiepileptic drug, which can be useful in the treatment of epilepsy. The primary site of action appears to be the motor cortex where the spread of seizure activity is inhibited. Phenytoin reduces the maximal activity of brain stem centers responsible for the tonic phase of tonic-clonic (grand mal) seizures. Steady-state therapeutic levels are achieved at least 7 to 10 days after initiation of therapy with recommended doses of 300 mg/day.


Phenytoin administration has been associated with toxic effects. Phenytoin toxicity depends on the route of administration, duration, exposure, and dosage. The route of administration is the most important determinant of toxicity. Phenytoin may be administered orally or intravenously. In addition, fosphenytoin may be administered intramuscularly. Blood levels of phenytoin reflect only the total serum concentration of the drug. Only the free unbound phenytoin has biological activity. Population groups that are predisposed to elevate free phenytoin levels include neonates, elderly persons, and individuals with uremia, hypoalbuminemia, or hyperbilirubinemia. These patients may exhibit signs of toxicity when drug levels are within the therapeutic range. Hepatic microsomal enzymes primarily metabolize phenytoin. Much of the drug is excreted in the bile as an inactive metabolite, which is then reabsorbed from the intestinal tract and ultimately excreted in the urine. Less than 5% of phenytoin is excreted unchanged in the urine. Individuals with impaired metabolic or excretory pathways may exhibit early signs of toxicity.