Bioavailability of oral and intramuscular phenobarbital

The absorption of phenobarbital was compared in healthy adult subjects after oral and intramuscular therapeutic doses. Serum levels of phenobarbital were determined for 21 days after dosing by means of radioimmunoassay. Serum levels were similar from both dosage routes, with peak levels occurring at 1-3 hours after dosing and then declining slowly with an elimination half-life of about 90 hours. The overall efficiency of phenobarbital absorption from intramuscular doses was approximately 80 per cent of that from equivalent oral doses. Except in cases where oral dosing is not appropriate, there is no clinical advantage in giving phenobarbital intramuscularly to adult patients.