Viagra takes about 30 minutes to start working. 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.

A surprising new area of research is whether taking a small dose of Viagra every night works as a preventative to stave off impotence, just as people can take an aspirin a day to prevent heart attack.

Though you wouldn't think you'd get much out of Viagra while you're sleeping, it turns out that nighttime erections - most men get three or four every night - are crucial to maintaining potency. Because most men don't have several erections during the day, the nocturnal erection, which allows blood and oxygen to flow to the genitals, is nature's way of keeping the penis in working order. The theory is that anything that increases the intensity and duration of nighttime erections is good for long-term potency.