Active substances that inhibit CYP3A4 activity such as macrolide antibiotics (e. g. erythromycin), azole antifungal agents (e. g. ketoconazole, itraconazole) or certain protease inhibitors (e. g. ritonavir) may increase the bioavailability of fentanyl by decreasing its systemic clearance, potentially enhancing or prolonging opioid effects.