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