Q. How does Viagra work?

A. Viagra belongs to a group of medicines called phosphodiesterase type 5 or PDE-5 inhibitors. It works to relax the muscle that can block blood flow to the penis. Without this blood flow there is no erection. So if this muscle is not regulated, you get no ejection.Viagra works by dilating blood vessels in the penis, allowing the inflow of blood needed for an erection. Viagra causes erections only during sexual excitement. It does not work in the absence of arousal.

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 MagicBluePill.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.

Because Viagra doesn't increase desire, it's not prescribed for men with desire disorders, such as a low sex drive. However, if the man has lost interest in sex because he has had problems with erections in the past, Viagra may help.

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