Both sildenafil and the metabolite have terminal half lives of about 4 hours. If you become pregnant while taking sildenafil, call your doctor. John’s wort. Population pharmacokinetic data from patients in clinical trials also indicated a reduction in sildenafil clearance when it was co-administered with CYP3A4 inhibitors such as ketoconazole, erythromycin, or cimetidine [see Dosage and Administration 2. This increased blood flow can cause an erection. Two of the questions from the IIEF served as primary study endpoints; categorical responses were elicited to questions about 1 the ability to achieve erections sufficient for sexual intercourse and 2 the maintenance of erections after penetration. Do not take a double dose to make up for a missed one.Examples of Viagra’s more commonly reported side effects include: headache. flushing (temporary warmth, redness, or deepening of skin color) stuffy nose. indigestion (upset stomach) mild vision. Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in adults (Liqrev, Revatio) and children 1 year of age and older (Revatio) with pulmonary arterial hypertension (PAH; high blood pressure in the vessels carrying blood to the . Viagra (sildenafil) is a prescription drug that’s used to treat erectile dysfunction (ED). The drug comes as a tablet you’ll take by swallowing. It’s usually taken as needed about 1 hour.