 - Ongoing therapy with citochrome P450 3A4 CYP3A4 inhibitors ketoconazole, erythromycin, cimetidine, clarithromycin, itraconazole, voriconazole and protease inhibitors or CYP3A4 inductorscarbamacepine, phenytoin, phenobarbital, rifampicin, Sint Janskruid . Furthermore patients will be informed not to drink grapefruit juice while on study medication because of the known impact of grape fruit on pharmacokinetics of Sildenafil. 