Keep track of Closely (1)lenacapavir will increase the level or effect of fentanyl by affecting hepatic/intestinal enzyme CYP3A4 metabolism.Keep an eye on Closely (2)efavirenz will lower the level or effect of fentanyl by affecting hepatic/intestinal enzyme CYP3A4 metabolism. Modify Therapy/Observe Intently. Coadministration of fentanyl with CYP3A4