pentobarbital will reduce the level or influence of clarithromycin by affecting hepatic/intestinal enzyme CYP3A4 metabolism. Insignificant/Significance Unfamiliar. pentobarbital will reduce the level or outcome of tamoxifen by influencing hepatic/intestinal enzyme CYP3A4 metabolism. Use Caution/Monitor. Contraindicated. CYP3A4 is to blame for