How does Viagra work? And what exactly does it do? Does it just make you really horny? Will it increase sexual pleasure? Will it enable one to maintain an erection for a longer period of time, or shorten “resting” time between erections?
http://www.viagra.com/ might be able to help you with your problem.
Scientific American had an article recently on male sexual, um, mechanics and mentioned the specific mechanism that Viagra uses. I don’t recall the details at the moment but they emphasized that it did not involve those portions of the brain that control arousal, etc., but was purely an effect of the spinal cord and nerves of the penis. They contrasted Viagra with another soon-to-be-available medication that did involve the brain. At least as much as it is ever involved in male sexuality.
The article did state that the sympathetic nervous system (the automatic part, that controls breathing, etc.) actively prevents erections. During REM sleep that portion of the sympathetic nervous system sometimes shuts down and the result is erection while sleeping, also known as “morning wood”, although the article didn’t use that term.
IIRC, nitric acid or something similar is involved.
Viagra, which is sildenafil, inhibits an enzyme involved in blood flow to the penis.
Specifically, blood flow to the corpus cavernosum and corpus spongiosum (i.e. those parts of the penis that become engorged with blood to cause an erection) is controlled by the level of a substance called cyclic guanosine monophosphate (cGMP). cGMP causes the arteries to dilate and allow more blood through. Thus, the more cGMP, the more blood flow in, the better the erection.
Normally, cGMP is broken down by an enzyme called phosphodiesterase type 5. So, if you can turn off this enzyme, you can increase/maintain blood flow into the penis.
Viagra (Sildenafil) inhibits phosphodiesterase type 5.
Note that cGMP has an effect on blood vessels and blood flow other than just in the penis. So, artificially high levels of cGMP, such as occur with Viagra use, can dilate other blood vessels and cause low blood pressure (if you increase the diameter of a pipe, the pressure within will drop).
Have fun!