Many men suffer from erectile dysfunction (ED) -- the inability to achieve and sustain an erection long enough to have enjoyable and fulfilling sex. For a variety of reasons ED is most common in men over 45. The sex drive is often lowered even further in older men who are taking medications of various kinds.

Most of us assume the sex drive gradually diminishes as we get older, so we are prepared to accept ED as a natural condition. But the fact is, the inability for many men to sustain an erection may be more the result of treatable physical conditions rather than an actual loss of sex drive.

Until the early 1980s most people -- including most trained urologists -- assumed that ED was basically a mental or psychological problem. But in 1983 at a presentation to the American Urological Association, Dr. Giles Brindley demonstrated that a penis could be made erect by injecting it with the drug phentolamine. In a public demonstration, he used his own penis to make the point.

Brindley had discovered that the penis could be made erect by relaxing the normally consricted blood vessels leading into it. Once the blood vessels are relaxed, they let in more blood and the penis inflates something like a balloon to form an erection.

So gradually it came to be accepted that ED is the result of physical changes in the adult male -- not primarily mental or psychological conditions.

Viagra - Controlling the Penile Valve

Phentolamine -- the drug used by Dr. Brindley to give himself a public erection -- started to be used with some success in the 80's and 90's, but there are two problems with phentolamine. First, it is not selective enough to target only the penis, so its effects on other parts of the body are unpredictable. And second, the erections it creates are not brought on by sexual stimulation. You can get an erection any old time with phentolamine, and you will continue to have one until the effect of the drug wears off.

Viagra was the first drug that overcame these two problems. (It was introduced in 1998 by the Pfizer company. The active ingredient in Viagra is sildenafil citrate.) Viagra does not work by relaxing the blood vessels of the penis. Instead it enhances the natural processes that take place when a man is sexually stimulated. It does this by controlling what we might call the "Softeners" -- the chemicals in the body designed to make the penis go soft after an erection.

It works something like this. When a man is sexually stimulated his brain sends signals that trigger the production of chemicals that relax the blood vessels in the penis. Think of these as "Hardeners". As the blood vessels are relaxed by the production of Hardeners, blood flow increases and the penis goes erect.

But nature has also designed a way for the erection to gradually subside. At the same time as Hardeners are being produced to relax the blood vessels, the body is also producing Softeners to break down and neutralize the Hardeners. These are an enzyme known as phosphodiesterase (PDE5).

As long as the sexually stimulated male is producing more Hardeners than Softeners, the penis will remain erect. But as soon as the production of Hardeners starts to go down, the Softeners will get the upper hand, and the erection will start to fizzle.

It is now generally assumed that ED is caused by an imbalance of Hardeners and Softeners. The body is either not producing enough Hardeners, or too many Softeners. Either way, the result is the inability to sustain an erection.

What Viagra does is block the production of Softeners. This allows the Hardeners to build up in the blood vessels of the penis and create hard, sustained erections.

The beauty of the process is that nothing happens without sexual stimulation. Viagra does not take the place of stimulation. Instead, it simply lets the natural chemicals created by sexual stimulation do their work. After sex, when the brain is no longer creating Hardeners, the erection subsides naturally. And when the effects of Viagra wear off -- normally after 4 or 5 hours -- the normal processes are restored to the way they were before taking the drug.

Of course, as with any drug, there are potential side effects. Viagra should not be taken if you are taking any nitrate-based drug, including nitroglycerin or if you are on any blood pressure or heart medication, as combining Viagra with these drugs can cause a severe drop in blood pressure. Also, if you have any liver or kidney problems it is not recommended that you take Viagra.

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Rick Hendershot is a writer and online publisher. For online promotion ideas, see Power Listings.