Pregnancy increases reactivity of mice to phenobarbital.

Compared to nonpregnant controls, pregnant mice injected with phenobarbital had lower concentrations of the drug in the plasma but equivalent concentrations in the brain. In spite of the similar concentrations in the brain, the behavioral response to phenobarbital was greater for pregnant than nonpregnant mice. These results suggest that the concentration of phenobarbital in the plasma, which is commonly used as a basis for adjusting phenobarbital dosage during pregnancy, is not an appropriate indicator of the dynamics of the drug.