The effect of phenobarbital on the metabolism of meperidine in normal volunteers.

Phenobarbital has been observed clinically to alter the metabolism of meperidine, with resultant enhanced toxicity. In order to determine if this effect occurs consistently, 12 health volunteers were entered into a two-way, crossover study comparing the pharmacokinetics and metabolism of meperidine after pretreatment with both phenobarbital and placebo. Phenobarbital pretreatment had no significant effect on serum levels or the half-life of meperidine. However, phenobarbital pretreatment resulted in a decrease in the cumulative excretion of meperidine and an increase in the cumulative excretion of the N-demethylated metabolite normeperidine. Similarly, phenobarbital pretreatment resulted in a decrease in meperidinic acid and increase in normeperidinic acid. In addition, phenobarbital pretreatment also significantly altered the hepatic clearance of meperidine, indicating an increase in the hepatic N-demethylation of meperidine. Since normeperidine has been reported to be less efficacious and more toxic than meperidine, this reported interaction may be important clinically, especially with repeated doses.