If it is not treated right away, priapism can permanently damage your penis sudden vision loss in one or both eyes. There are many other treatments for erectile dysfunction. Sildenafil works on the PDE5 enzyme in the lungs to relax the blood vessels. Resources and support See the Healthy Male site for more information about erectile dysfunction and its treatment. Sildenafil belongs to a group of medicines called phosphodiesterase 5 PDE5 inhibitors.Viagra (sildenafil) and cost. Viagra (sildenafil) is a brand-name oral tablet prescribed for erectile dysfunction. The cost of the drug with and without insurance can depend on several factors. Sildenafil (Viagra) is a medicine used to treat erectile dysfunction (ED, also called impotence). Sildenafil works by helping to relax the blood vessels in the penis, allowing more blood to flow in and cause an erection. You should take sildenafil about an hour before planned sexual activity, so it has time to work. VIAGRA can cause your blood pressure to drop suddenly to an unsafe level if taken with certain other medicines. Do not take VIAGRA if you take any other medicines called “nitrates.” Nitrates are used to treat chest pain (angina). A sudden drop in blood pressure can cause you to feel dizzy, faint, or have a heart attack or stroke.