Erectile dysfunction (ED) occurs when a man has difficulty initiating or maintaining an erection firm enough for sexual intercourse. The National Institutes of Health estimates that ED affects as many as 30 million men in the United States.
An erection begins with sensory or mental stimulation, or both. Impulses from the brain and local nerves cause the muscles of the penis to relax, allowing blood to flow through the arteries. The penis then expands in size and maintains its rigidity by preventing the outflow of blood. The erection ends after orgasm and ejaculation as blood drains out through the penile veins.