Why is this medication prescribed?

Alprostadil injection and suppositories are used to treat certain types of erectile dysfunction (impotence; inability to get or keep an erection) in men. Alprostadil injection is also sometimes used in combination with other tests to diagnose erectile dysfunction. Alprostadil is in a class of medications called vasodilators. It works by relaxing the muscles and blood vessels in the penis to keep enough blood in the penis so that an erection can occur.
Alprostadil does not cure erectile dysfunction or increase sexual desire. Alprostadil does not prevent pregnancy or the spread of sexually transmitted diseases such as human immunodeficiency virus (HIV).