Sildenafil citrate is a prescription medication which is available as a generic drug and under the brand name Viagra. Sildenafil is a member of the phosphodiesterase inhibitor family. Here's how sildenafil works: When a man becomes erect, the arteries in his penis carry blood through the spongy tissue inside. When a health problem keeps genital blood from flowing properly, sildenafil increases blood flow to the penis, resulting in an erection. Sildenafil has a four-hour duration of action, meaning that sex play can last for some time. Viagra is available in twenty-five, fifty and one-hundred milligram tablets. Sildenafil should be taken as needed about thirty to sixty minutes before sexual activity. For maximum effectiveness, it should be taken on an empty stomach or with a low-fat meal.