Short Version:  grapefruit juice contains enzymes that change absorbtion rates of compounds (Viagra, in this case) in the intestine.

Long Version: Researchers have found that the absorption of Viagra increased by 23% when taken with grapefruit juice instead of water (sounds good, right?  don't be so sure...see below). However, grapefruit juice also delayed the absorption of Viagra. This latter point is important because Viagra is supposed to be taken one hour before sex, and taking the drug with grapefruit juice may result in disappointment for some users of Viagra.

Men who use protease inhibitors are usually prescribed less-than-normal doses of Viagra because protease inhibitors can raise levels of Viagra several times greater than normal, which can cause dangerous side effects. Therefore, men who use protease inhibitors and Viagra may wish to also avoid taking Viagra with grapefruit juice.