Phenytoin Poisoning

Basics

Description

  • Phenytoin follows zero-order pharmacokinetics at therapeutic and supratherapeutic concentrations:
    • Small incremental increase in dose can result in a large increase in plasma concentration
  • Half-life in overdose prolonged; may be up to 70 hr
  • Cardiovascular toxicity from IV administration likely due to the diluent propylene glycol
  • Fosphenytoin, a prodrug for parenteral administration, lacks propylene glycol and is metabolized to phenytoin, its active moiety

Etiology

  • Phenytoin intoxication results from acute, chronic, or acute-on-chronic administration
  • If the cause of the intoxication is unclear in a patient receiving chronic phenytoin therapy, consider that there may have been a:
    • Change in the brand of phenytoin
    • Change in dosage form
    • Drug interaction
    • Change in serum albumin

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