The term erectile dysfunction can mean the inability to achieve erection, an inconsistent ability to do so, or the ability to achieve only brief erections.Some men, however, experience constant, complete erectile dysfunction (impotence), and others, partial or brief erections. Since Viagra is only for those who have erectile dysfunctions, the drug must be taken orally once a day, one hour before the sexual activity. The exact reason behind your problem can only be judged by a specialist and only then one should use the medicines prescribed. You can have the information on ED and ED related drugs such as generic Viagra on
http://www.pillsguide.net/group/generic_viagra
So go out, consult an urologist.