pentobarbital will decrease the level or effect of oxybutynin by affecting hepatic/intestinal enzyme CYP3A4 metabolism. Minor/Significance Unknown.pentobarbital will decrease the level or effect of fentanyl ort affecting hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Skärm Closely. Coadministration of fentanyl with CYP3A4 inducers cou