Erection problems (erectile dysfunction) may be caused by physical problems related to the blood vessels, nerves, and hormones or by psychological issues. Current evidence suggests that physical causes account for about 80% of erection problems.¹

Normally, an erection occurs when your imagination or senses (vision, hearing, touch, smell, taste) are stimulated and you become aroused. Your central nervous system sends nerve impulses that increase blood flow to your penis. Blood fills the spongy chambers (corpora cavernosa) in the penis, causing them to expand and become rigid.

Four requirements for a normal erection are:

• A properly functioning nervous system that sends the necessary signals to the penis.
• An intact system of blood vessels (vascular system) to allow blood to flow into and out of the penis.
• Normal smooth muscle in the penis, which must relax so the penis can fill with blood and enlarge.
• The ability to trap the blood in the penis so that it stays firm.

Physical causes of erection problems include long-term (chronic) and short-term (acute) injuries and complications of prostate or other surgery that interfere with nerve impulses or blood flow to the penis. Physical problems are often the cause of erection difficulties in men age 50 or older.

• Problems with the blood vessels (vascular problems) may prevent blood from filling the penis or from remaining there long enough to maintain an erection. For example, long-term high blood pressure can cause damage to blood vessels and lead to erection problems.
• Problems with the nerves (neurologic problems) may prevent arousal signals from traveling from the brain and spinal cord to the penis. Nerve disorders such as Parkinson's disease, Alzheimer's disease, multiple sclerosis, and stroke may interfere with a man's ability to have an erection and may lower sexual desire. Nerve damage from diabetes, complications from surgery, and spinal cord injury also may cause problems.
• Problems with the structure of the penis or its surrounding tissues may prevent an erection.
• Hormonal factors, such as a low level of the hormone testosterone, may be involved in causing erection problems.
• Side effects of medications (for example, some of those taken for high blood pressure or depression) may include erection problems. In some cases it may be possible to change the dose of the medicine or to use another medicine.
• The use of tobacco, alcohol, or illegal drugs can lead to erection problems. Stopping or reducing the use of these substances may reduce the severity of a man's erection problem.

Activities that restrict blood flow to the penis also may result in erection problems. Some doctors have observed that men who regularly ride bicycles over long distances are more likely to have erection problems than men who ride only occasionally, especially if they ride on a narrow, unpadded saddle. But the possible link between bicycle riding and erection problems has not been proven.