Bioavailability of Oral and Intramuscular Phenobarbital

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Abstract
The absorption of phenobarbital was compared in healthy adult subjects after oral and i.m. 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 h after dosing and then declining slowly with an elimination half-life of about 90 h. The overall efficiency of phenobarbital absorption from i.m. doses was .apprx. 80% of that from equivalent oral doses. Except in cases where oral dosing is not appropriate, there is no clinical advantage in giving phenobarbital i.m. to adult patients.