Pharmacokinetics and relative bioavailability of intramuscular phenobarbital sodium or acid in infants.

Phenobarbital pharmacokinetics after a single intramuscular dose of approximately 10 mg/kg of phenobarbital sodium or acid were studied in twelve infants with febrile convulsions. The mean peak time after phenobarbital sodium (0.92 hr) was significantly (p less than 0.01) shorter than that after phenobarbital acid (5.83 hr), whereas the mean peak levels (16.2 vs 13.8 microgram/ml) did not reach statistical difference (p less than 0.2) between both dosage forms. The values of the absorption rate constant and the area under the curves calculated from 0- to up to 6-hr periods were significantly higher (P less than 0.001 to 9.005) after the sodium form. There were no significant differences in such parameters as t1/2, Vd/F, and C1/F between the two preparations. The overall mean values of these variables were: t1/2, 64.0 hr, Vd/F, 0.667 1/kg, and C1/F, 0.136 m1/min/kg. Some delay in rising phenobarbital concentration in cerebrospinal fluid (CSF) as compared with that in plasma after phenobarbital sodium was observed, though the absolute values of phenobarbital concentrations in CSF were similar from both dosage forms around the time when peak plasma levels were attained.