If you suffer from erectile dysfunction (impotence), you're hardly alone. It is estimated that about half of all men between ages 40 and 70 in the U.S. suffer from erectile dysfunction (ED) to some degree. The good news is that ED can nearly always be treated effectively. Much of this success can be attributed to the oral treatment sildenafil citrate (brand name Viagra), the most widely used treatment for ED. It works by enhancing the effects of one of the chemicals the body normally releases into the penis during sexual arousal. This allows an increase of blood flow into the penis, causing an erection. More than four patients in five taking Viagra experience an improvement in their erections. Viagra must never be taken by men tho take any nitrate medicine, such as nitroglycerine (used for the treatment of chest pains). To do so risks a drop in blood pressure to life-threatening levels.