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  1. Oldest of the nonsedating antiepileptic drugs
  2. Has effect on several physiologic systems; alters Na, K and Ca conductances, membrane potentials, and concentrations of amino acids, blocks sustained high frequency repetitive firing of action potentials
  3. Elimination of phenytoin is dose dependent; as blood levels rise within the therapeutic range, the maximum capacity of the liver to metabolize phenytoin is approached with small changes in dose producing large changes in phenytoin concentration
    1. thus ½ life can range form 12-36 hours with an average of 24 hours for most patients in the low to mid therapeutic range
  4. decreases in plasma protein concentration result in increased levels of free phenytoin and may cause intoxication
    1. Side effects: 1. CNS (nystagmus, ataxia, confusion, dizziness, asterixis, insomnia, dyskinesias), 2. sensory peripheral neuropathy, 3. GI (nasusea, vomiting, constipation, liver toxicity), 4. systemic (fever, rash, tocix necrotic epidemolysis, Steven-Johnson sd), 5.connective tissue ( coarse facial features, gingival hyperplasia, hypertrichosis, gynecomastia)
  5. phenytoin blood levels may be increased or by chloramphenicol, cimetidine, coumadin, isoniazid, and sulfonamides
  6. phenytoin blood levels may be increased or decreased by phenobarbital, valproic acid, or valproate
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