Decreased phenobarbital absorption with charcoal administration for chronic renal failure

To report a case of impaired absorption of orally administered phenobarbital associated with the concomitant administration of activated charcoal, and recovery of the absorption after administration of the 2 drugs was separated by a 1.5-hour interval.

A 78-year-old woman, weighing 50 kg, who had undergone brain surgery was prescribed phenobarbital 120 mg/day for postoperative convulsions. Her serum phenobarbital concentration reached 24.8 microg/mL (therapeutically effective level 10-30). Thereafter, her renal function worsened, and activated charcoal 6 g/day was started. Four months after the start of activated charcoal, blood analysis revealed that the serum phenobarbital concentration was as low as 4.3 microg/mL. The phenobarbital dose was increased to 150 mg/day. Further evaluation revealed that activated charcoal and phenobarbital had been administered concomitantly. The dosage regimen was altered to separate the administration of the agents by at least 1.5 hours. Subsequently, the patient's serum phenobarbital concentration increased to 11.9 microg/mL within 3 weeks. Her serum phenobarbital concentration was measured monthly thereafter and remained stable in the range of 14.8-18.6 microg/mL.

Our patient's low serum phenobarbital concentration was considered likely to have been due to impaired gastrointestinal absorption of phenobarbital as a result of adsorption of phenobarbital on the activated charcoal. An objective causality assessment showed that the interaction was probable.

Administration of activated charcoal and phenobarbital should be separated by an interval of at least 1.5 hours.