When sexually stimulated there is a release of a chemical, nitric oxide (NO) in the blood vessels of the corpus cavernosum. The NO stimulates the production of a compound called cGMP, which causes relaxation of the smooth muscle in the blood vessels supplying the corpus cavernosum. PDE 5 is an enzyme that breaks down cGMP. By inhibiting the breakdown of cGMP by PDE5, these medications allow cGMP to build up in the penis. cGMP causes muscles in the corpora cavernosa of the penis to relax. When the muscle is relaxed, more blood can flow into the penis and fill the spaces in the penis. As the penis fills with blood, the veins in the penis are compressed, and this results a hard erection. When the effect on PDE5 decreases, the cGMP levels go down and the muscle in the penis contracts, causing less blood to flow into the penis and allowing the veins to open up and drain blood out of the penis.
Prior to starting with treatment of erectile dysfunction, it is important to make sure that it is safe from a medical standpoint to participate in sexual activity. Sexual activity is physical exertion, and in some men with significant heart disease, this increase in physical exertion can increase the risk of a heart attack. Thus, it is very important to discuss your cardiovascular risks with your doctor prior to trying any medication or treatment for erectile dysfunction.
The art of acupuncture has become the new treatment for everything from back pain, depression, and even ED. Impotence could be more of a state of mind, and acupuncture may help. Through this alternative therapy, fine needles are placed in various parts of the body to relieve pain or stress. Although there are many mixed studies for acupuncture and ED, many tend to confirm positive results. A 1999 study found acupuncture improved the quality of erection and even restored sexual activity in 39 percent of participants.
Trauma to the pelvic blood vessels or nerves can also lead result in ED. Bicycle riding for long periods has been implicated as an etiologic factor; direct compression of the perineum by the bicycle seat may cause vascular and nerve injury.  On the other hand, bicycling for less than 3 hours per week may be somewhat protective against ED.  Some of the newer bicycle seats have been designed to diminish pressure on the perineum. [37, 38]
Injection therapy: The modern age of such drug therapies began in 1993 when the injection of papaverine (Pavabid), an alpha-blocker that produces vasodilatation (widening of the blood vessels), was shown to produce erections when injected directly into the penis. Soon afterward, other vasodilators, such as prostaglandin E1 (PGE 1) monotherapy (Caverject, Edex), PGE1 and phentolamine (Regitine), and Trimix (papaverine, phentolamine and prostaglandin E1), were demonstrated to be effective. The benefit of combination therapy is the decreased dosing of each with less side effects. Most important is the reduction of the prostaglandin PGE1 dosing, which is associated to the localized pain.
There are, as you listen to all of the advertisements, if your erection lasts for more than four hours, there are very, very unusual cases where that can happen. There are very rare cases of visual problems. There are even rarer cases of hearing problems. But with every medication, there always a potential downside. But the absolute contraindication is an unstable medical condition, an unstable cardiovascular condition, being on nitrates.