Combined administration of carbamazepine and phenobarbital: effect on anticonvulsant activity and neurotoxicity.

Despite the trend towards single drug therapy of epilepsy, patients resistant to monotherapy are commonly treated with more than one antiepileptic drug. As part of an investigation on the experimental background for antiepileptic drug combinations, the effect of the pharmacodynamic interactions between carbamazepine and phenobarbital on the toxicity/efficacy ratio was studied in mice. All results were expressed in terms of drug concentrations in the brain to exclude possible pharmacokinetic interactions from the analysis. A purely additive interaction was found for the anticonvulsant as well as for the neurotoxic effect of the drugs. The combination of carbamazepine and phenobarbital has therefore no advantage over each drug alone in this model. Based on these and previous results, there is no experimental evidence in favor of any combination between the four main drugs against partial seizures, i.e., carbamazepine, phenytoin, phenobarbital, and primidone.