Diminished mephobarbital anticonvulsant action following diphenhydramine pretreatment.

Diphenhydramine and other antihistamines produce biphasic effects on drug disposition and lower seizure threshold, thereby potentially diminishing the efficacy of anticonvulsants such as mephobarbital. Accordingly, the influence of diphenhydramine (50 mg/kg, IP) pretreatment on the anticonvulsant activity of mephobarbital (50 mg/kg, IP) was determined in adult female Swiss-Webster mice given pentylenetetrazol (SC). Diphenhydramine lowered the pentylenetetrazol convulsive dose (CD50) by 60%. Administration of diphenhydramine in combination with mephobarbital produced a 65% decrease in the CD50 of pentylenetetrazol in comparison with that of animals given mephobarbital plus pentylenetetrazol. Pharmacokinetic evaluation of mephobarbital blood level data indicates that the mechanism responsible for the observed interaction between diphenhydramine and mephobarbital involves a decrease in mephobarbital uptake from the administration site.