Approximately 25% of an administered dose of phenobarbital is cleared by the kidney as unchanged drug. It is often necessary to lower phenobarbital dosage in patients with renal insufficiency, but there are no clear guidelines for this dosage adjustment.59 Patients with renal insufficiency taking phenobarbital should have their dosage adjusted as dictated by phenobarbital plasma concentration determinations.
Phenobarbital is sparingly soluble in water, is approximately 50% protein bound, and has a modest volume of distribution. Significant amounts of drug are lost during both hemodialysis and peritoneal dialysis. Supplemental phenobarbital after dialysis is often required,59,60 but there are no clear guidelines as to how much is needed. The loading-dose requirement should be determined by measuring the post-dialysis phenobarbital plasma concentration and employing the equation shown in Correction for drug loss during hemodialysis. The assumed volume of distribution is 0.7 L/kg.
Reviewed and revised February 2004 by Steven C. Schachter, MD, epilepsy.com Editorial Board.
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