• Eran Kozer, Shahid Parvez, Berge A Minassian, Jeff Kobayashi, Zul Verjee, and Gideon Koren.
• Division of Clinical Pharmacology and Toxicology, Department of Pediatric Laboratory Medicine, The Hospital for Sick Children, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8. eran.kozer@sickkids.on.ca
• Ther Drug Monit. 2002 Jun 1; 24 (3): 386-9.

AbstractPhenytoin is an effective anticonvulsant, but high serum phenytoin concentrations may be associated with serious toxicity. The upper limit for the therapeutic serum concentration of phenytoin is considered to be 80 micromol/L. However, in some situations higher serum concentrations are needed to control seizures. The authors describe a 9-year-old girl who needed concentrations twice the normal amount to control recurrent episodes of decreased levels of consciousness. Except for nystagmus, she had no other signs of phenytoin toxicity. This patient highlights the critical principle in therapeutic drug monitoring of individualizing drug therapy. Although some patients receiving phenytoin may achieve seizure control with "subtherapeutic" levels (i.e., <40 micromol/L), others may need supratherapeutic levels, as was the case with this patient. Clinicians should be careful not to treat "numbers" (i.e., serum concentrations), but rather the patient's clinical condition, with a careful balance between therapeutic advantage and adverse effects.

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