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So, from doctors who prescribe it and people who use it, here are the answers to the Viagra questions many people wonder about -- but rarely ask.

What Happens When a Man First Takes a Viagra Pill?

Absolutely nothing. Pfizer Inc., the maker of Viagra, long has said the drug isn't an aphrodisiac, but many men who take it still expect to feel something.

They don't. Among several men interviewed who have used the drug, not one of them experienced any feeling or sensation after taking the pill. The nothingness is so intense that the most common reaction is a slight panic that the drug isn't going to work.

"That was my worst fear, that it wasn't going to do anything," says Steve Brykman of Los Angeles, who tried Viagra once nine months ago, when he believed job and financial stresses were interfering with his sex life. After taking the pill, "there was nothing at all," says Mr. Brykman, 33 years old. "I just felt completely normal."

Though you may not feel anything, things are happening in the body. As the pill moves into the bloodstream, it starts to block an enzyme called PDE-5. Blocking the enzyme eventually increases blood flow to areas where PDE-5 is most heavily concentrated -- the penis, nose and skin. Diminished blood flow to the penis is the cause of most erectile-dysfunction problems.

So How Do You Get It to Start Working?

Viagra gets the blood flowing, but your brain has to be in the mood as well. "The biggest misperception is that it changes your psychology and makes you want sex," says the editor of PillSamples.com (http://www.pillsamples.com), which claims to document the experiences of real people who take Viagra. "But if you're sitting talking to Grandma and you pop a Viagra, unless you have issues, nothing's going to happen."

Viagra takes about 30 minutes to kick in. Men who don't normally have problems, or who have only mild dysfunction, say it takes only a minor stimulus -- such as the brush of a hand that wouldn't cause arousal under normal circumstances -- to trigger an erection.

For men who have serious erectile dysfunction, getting things going may still require extra effort, partly because of nervousness or embarrassment about unsuccessful past attempts at intercourse.