Erectile dysfunction, also known as impotence, is an inability to achieve or maintain an erection that is firm enough to have sex. Erections depend on many different factors, including proper blood flow to the penis. Reduced blood flow to the penis may cause erectile dysfunction. Erectile dysfunction can affect men differently. Some cannot achieve an erection. Others may achieve an erection, but not firm enough for having sex or they may lose the erection before or during sex.