In vitro and in vivo effects of promethazine (Phenergan) on drug metabolism.

Prolongation effects of promethazine on the pentobarbital sleeping time are not due to interactions of this drug with cytochrome P-450 or cytochrome c reductase or inhibition of drug metabolism because pentobarbital plasma levels in promethazine treated animals before awakening are not different than in controls. Results suggest additive effects of both drugs on the central nervous system. Those interactions do however play a role during in vitro studies.