The effects of Viagra last from four to six hours, during which period you should be able to get and keep an erection hard enough for penetration. The recommended starting dose for Viagra is 50 milligrams, but some men may need as much as 100 milligrams to achieve the desired results. After the four- to six-hour period of optimal effectiveness, traces of the drug remain in your system.
Taking another dose within a 24-hour period could combine with this residual amount and result in dangerously low blood pressure, much as one might see from taking Viagra with a nitrate-based drug. For this reason, it’s wise to follow the directions and limit yourself to one dose per day.