<p>Concomitant use of Sildenafil with organic nitrates for angina may cause hypotension. Cimetidine, a medicine used to treat gastric ulcers, some antibiotics including Erythromycin and Rifampicin, some protease inhibitors such as Ritonavir and Saquinavir for the treatment of HIV infection may increase the plasma concentration of Sildenafil. Some medicines used to treat fungal infections including Ketoconazole and Itraconazole may reduce the clearance of Sildenafil.</p>