Penegra 50 mg contains sildenafil 50 mg. Medical advice is particularly important before using it if you: Take nitrates such as nitroglycerin/GTN or isosorbide. Sildenafil must not be combined with these because it can cause a dangerous drop in blood pressure. D DailyMed +1 Take riociguat; sildenafil is contraindicated with this medicine. D DailyMed Have significant heart or cardiovascular disease, particularly if your doctor has said ***** activity is unsafe. D DailyMed Take alpha-blockers or several blood-pressure medicines, because sildenafil can further lower blood pressure. D DailyMed Have severe kidney or liver impairment or are over 65; a lower starting dose may be appropriate. D DailyMed Take medicines that strongly increase sildenafil levels, such as ritonavir or certain CYP3A4 inhibitors; dose adjustment may be needed. D DailyMed Have previously had an allergic reaction to sildenafil. For most adults with ED, 50 mg is the usual starting dose, but the dose may be reduced to 25 mg or increased to 100 mg based on effectiveness and tolerability. It should generally be taken no more than once in 24 hours. D DailyMed