Tell your doctor if any of these symptoms are severe or do not go away: headache. Do not stop taking sildenafil without talking to your doctor. Vardenafil usually works for 4 to 5 hours. Sildenafil belongs to a group of medicines called phosphodiesterase 5 PDE5 inhibitors. If you have certain health conditions or are taking certain medications, your doctor may tell you to take sildenafil less often.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 before.  · Descriptions. Sildenafil is used to treat men who have erectile dysfunction (also called sexual impotence). Sildenafil belongs to a group of medicines called phosphodiesterase 5 (PDE5) inhibitors. These medicines prevent an enzyme called phosphodiesterase type-5 from working too quickly. The penis is one of the areas where . Viagra is used to treat erectile dysfunction (ED). This is unlike Revatio, which is used to treat pulmonary arterial hypertension. (This is a condition that causes high blood pressure in your.