Tadalafil may potentiate the hypotensive effects of organic nitrates, and is therefore contraindicated in patients receiving such drugs. Concomitant administration of Tadalafil with drugs that inhibits the cytochrome P450 isoenzyme CYP3A4 such as cimetidine, erythromycin, itraconazole, ketoconazole, and HIV-protease inhibitors may reduce Sildenafil clearance necessitating a reduction in dosage. However, plasma concentrations of Tadalafil are significantly increased by ritonavir, requiring even greater dosage reduction, and is therefore strongly recommended that these two drugs are not given together.