Take sildenafil at around the same times every day. Oral medicines for ED usually work well and cause few side effects. Sildenafil comes as a tablet and suspension liquid to take by mouth. An inherited eye disease that affects the retina. What side effects can this medication cause? That means they can increase the amount of Viagra in your body, which can cause unwanted side effects. Oral medicine for ED also might not be safe or work well if you have: Very low blood pressure, called hypotension, or uncontrolled high blood pressure, called hypertension.Viagra is usually taken only when needed, 30 minutes to 1 hour before sexual activity. You may take it up to 4 hours before sexual activity. Do not take Viagra more than once per . Sildenafil may be used to treat erectile dysfunction (ED or impotence) in men (Viagra brand) or pulmonary arterial hypertension in both men and women (Revatio brand), depending . 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 .