[Bio-availability of phenytoin free-acid serum level determinations whilst using a lactose-based tablet-preparation (author's transl)].

Phenytoin serum levels were determined in 103 children during the steady state of phenytoin therapy using a radio-immuno-assay. 94% of the patients had been treated as outpatients, 54% received phenytoin in combination with other antiepileptic drugs. The preparation used was phenytoin as free acid in lactose tablets (Epilan D). A therapeutic serum level of 10-20 mcg/ml was reached by an oral intake of 6,1 +/- 1,6 mg/kg/d. 27 children had plasma levels above 20 mcg/ml, their oral dose was 7,1 +/- 1,8 mg/kg/d. Most of these children showed clinical signs of intoxication. The bioavailability of phenytoin preparations is not only depended on the type of the substance used (salt, free acid), but also on the excipients used for the preparation of tablets. Therefore, general dosage recommendations are useless and even dangerous, unless the preparations used are extensively specified.