OBJECTIVE: 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. CASE SUMMARY: 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 μg/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 μg/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 μg/mL within 3 weeks. Her serum phenobarbital concentration was measured monthly thereafter and remained stable in the range of 14.8-18.6 μg/mL. DISCUSSION: 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. CONCLUSIONS: Administration of activated charcoal and phenobarbital should be separated by an interval of at least 1.5 hours.
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