title: Phenytoin: the pseudosteady-state phenomenon.
abstract: After reaching an apparent steady state, plasma phenytoin (PHT) levels may then undergo inexplicable changes, a phenomenon called " pseudosteady state". We evaluated 13 pseudosteady -state periods in 10 inpatients with complex partial seizures. Eleven of the periods occurred after a change in PHT dosage and two after drug withdrawal. The pseudosteady -state period began 2 to 12 days (means = 5.7 days) after dosage change and lasted 5 to 10 days (means = 6.3 days), during which plasma PHT levels were stable (+/- 5%). Plasma PHT levels thereafter fluctuated spontaneously by greater than 25% for 5 to 22 days (means = 10.8 days). A final steady-state level was reached 13 to 31 days (means = 21.4 days) after the first dosage change. Falling plasma PHT levels increased seizure frequency in two patients, and a level of 52 micrograms/ml led to medication toxicity in another.
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D007700:Kinetics; D008297:Male; D008875:Middle Aged; D010672:Phenytoin / Q000097:blood* / Q000627:therapeutic use; D012640:Seizures / Q000097:blood / Q000188:drug therapy
chemical_list: D010672:Phenytoin