Effect of sodium valproate on phenobarbital serum levels in children and adults.

The influence of sodium valproate on serum levels of phenobarbital during combination treatment was studied in 29 children and 50 adults with epilepsy. Steady-state drug levels in serum were determined immediately prior to drug administration using immunoenzymatic analysis. The serum level/dose ratio of phenobarbital increased significantly (p less than 0.001) when sodium valproate was added to the treatment. The increase had a mean value of 50.9% in adults and 112.5% in children, suggesting marked interindividual variability in the intensity of the interaction. Almost half of the patients required a decrease in the dose of phenobarbital prescribed. The interaction was more pronounced in patients with high serum levels of phenobarbital, while the dose of phenobarbital and the serum levels and dose of sodium valproate did not seem to affect the extent of the interaction. Close monitoring of the serum levels of phenobarbital is recommended during simultaneous treatment with sodium valproate.