Q. What Happens When a Man First Takes a Viagra Pill?

A. 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.

Q. How does Viagra work?
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Q. How long does the effect of Viagra last?
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Q. Who should not take Viagra?