Ask your healthcare professional how you should dispose of any medicine you do not use. These are commonly prescribed for an enlarged prostate or male pattern baldness. About sildenafil Viagra Sildenafil is a medicine used to treat erection problems erectile dysfunction. Taking sildenafil alone will not cause an erection. However the dose is usually not more than 40 mg 3 times a day. Tadalafil Cialis.The active drug in Viagra is sildenafil. It belongs to a group of drugs called phosphodiesterase-5 inhibitors. Viagra comes as tablets you take by mouth. You’ll only need to take it as. 1. How it works. 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 on the brand. Sildenafil blocks the action of an enzyme known as cGMP-specific phosphodiesterase type-5 (PDE-5). What does Viagra do for ED? Viagra works to treat ED by helping you have and maintain an erection. The drug does this by improving blood flow into your penis when you’re sexually aroused.