Vardenafil and tadalafil belong to the same group of chemical compounds as sildenafil, namely phos-phodiesterase type 5 (PDE-5) inhibitors. Some men cannot benefit from sildenafil or the two newer PDE-5 inhibitors because they have low levels of nitric oxide. British investigators reported in late 2002 that three different types of compounds are being studied as possible medications for men with low levels of nitric oxide. They are Rho-kinase inhibitors, soluble guanylate cyclase activators, and nitric oxide-releasing PDE-5 inhibitors.
The role of the endothelium in ED has been noted for a number of years and the overlapping of ED and other conditions, especially coronary heart disease, CVD, affecting endothelial function/dysfunction, is clearly present. The endothelial cell is now known to affect vascular tone and impact the process of atherosclerosis, and impacting ED, CVD and peripheral vascular disease.16
Appallingly little is known about the prevalence of erectile dysfunction in the United States and how this prevalence varies according to individual characteristics (age, race, ethnicity, socioeconomic status, and concomitant diseases and conditions). Data on erectile dysfunction available from the 1940's applied to the present U.S. male population produce an estimate of erectile dysfunction prevalence of 7 million.
#9 Smokers hell. Bad stimulants such as smoking cigarettes may be having a negative impact on more than just your lungs. That’s right, smoking may lead to a lazy penis. One study shows that 40% of men that suffer from erectile dysfunction are smokers. The good news is that 75% of men reported erectile issues disappear after quitting smoking. What better motivation to quit smoking than having amazing sex? [Read: How to keep an erection up for longer in 20 ways]
3. Are there physical causes of erectile dysfunction? Erectile dysfunction may be a symptom of underlying medical conditions, which if not detected may cause further medical problems. A prior history of cigarette smoking, heart attacks, strokes, and poor circulation in the extremities (for example, intermittent claudication or cramping in your leg[s] when you walk) suggest atherosclerosis as the cause of the erectile dysfunction. Loss of sexual desire and drive, lack of sexual fantasies, gynecomastia (enlargement of breasts), and diminished facial hair suggest low testosterone levels. A prior history of pelvic surgery or radiation and trauma to the penis/pelvis/perineum can cause problems with the nerves and blood vessels. Symptoms of intermittent claudication of the lower extremities with exercise may suggest a vascular problem as a cause of the erectile dysfunction.
Cosgrove et al reported a higher rate of sexual dysfunction in veterans with posttraumatic stress disorder (PTSD) than in veterans who did not develop this problem.  The domains on the International Index of Erectile Function (IIEF) questionnaire that demonstrated the most change included overall sexual satisfaction and erectile function. [43, 44] Men with PTSD should be evaluated and treated if they have sexual dysfunction.
Having erection trouble from time to time isn't necessarily a cause for concern. If erectile dysfunction is an ongoing issue, however, it can cause stress, affect your self-confidence and contribute to relationship problems. Problems getting or keeping an erection can also be a sign of an underlying health condition that needs treatment and a risk factor for heart disease.