Phenytoin: pharmacokinetics and bioavailability.

The pharmacokinetics of a single 300-mg oral and intravenous and 14 daily 300-mg oral doses of phenytoin were studied in 6 healthy volunteers. The mean plasma elimination t1/2 was the same following intravenous (16.8 +/- 1.3 hr) and oral (17.1 +/- 1.5 hr) doses of phenytoin; however, following chronic oral administration, the t1/2 increased to 18.9 +/- 1.5 hr (p less than 0.05). The absolute bioavailability of an oral dosage form (Dilantin Kapseals) varied from 57.7 to 85.6% when based on the relationship between the corresponding single dose areas under the curve (AUCs). When based on the comparison of the AUC for multiple oral dosing with the single iv dose area, average bioavailability was 85.9% (71.8 to 106.3). Since the variation in the bioavailability and elimination of phenytoin does not allow accurate prediction of the steady-state plasma concentration, monitoring plasma levels may be of special importance.