Impotence, or erectile dysfunction, is the inability to achieve or maintain an erection long enough to engage in sexual intercourse.

Under normal circumstances, when a man is sexually stimulated, his brain sends a message down the spinal cord and into the nerves of the penis. The nerve endings in the penis release chemical messengers, called neurotransmitters, which signal the corpora cavernosa (the two spongy rods of tissue that run the length of the penis) to relax and fill with blood. As they expand, the corpora cavernosa close off other veins that would normally drain blood from the penis. As the penis becomes engorged with blood, it enlarges and stiffens, causing an erection. Problems with blood vessels, nerves, or tissues of the penis can interfere with an erection.

It is estimated that 10-20 million American men frequently suffer from impotence and that it strikes up to half of all...