Drug Image Gallery Pfizer makes Viagra. See more drug pictures.

Viagra is one of the best-known drugs of all time:

• Billions of spam e-mail messages advertise Viagra every day. There is so much Viagra spam, in fact, that Pfizer (the maker of Viagra) has a page addressing the problem, called Avoid Fake Viagra.

   

• Pfizer has spent untold millions of dollars advertising Viagra, so you see advertisements for the drug constantly on TV.

   

• Pfizer claims on its Web site that nine Viagra pills are dispensed every second -- nearly 300 million tablets per year.

What Viagra does is simple: When it works as intended, Viagra causes a man who is sexually stimulated to get an erection.

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­How does Viagra do that? And why does Viagra work only if the man is sexually stimulated? For that matter, what causes an erection in the first place? In this article, we'll answer all of those questions and more.

This is actually a fascinating story -- it involves the technology of the human body and the techniques that scientists use to control different parts of the body with drugs. And in the case of Viagra, the story starts with the technology of the penis.