Istradefylline forty mg/day amplified peak levels and AUC of CYP3A4 substrates in clinical trials. This effect was not observed with istradefylline 20 mg/day. Consider dose reduction of delicate CYP3A4 substrates.Concomitant use of fentanyl injection with CYP3A4 inducers or discontinuation of the CYP3A4 inhibitor could reduce fentanyl plasma concen