The problem is more common in men over age 65, but it can occur at any age. It is normal for men to experience changes in erectile function, such as taking longer to achieve an erection. The incidence of ED increases with age: About 5 percent of 40-year-old men and up to 25 percent of 65-year-old men experience ED, the NIDDK says. It is not an inevitable part of aging.

In older men, ED usually has a physical cause, such as disease, injury, or side effects of medication. Any disorder that causes injury to the nerves or impairs blood flow in the penis may cause ED, the NIDDK says. Damage to nerves, arteries, smooth muscles, and fibrous tissues, often as a result of disease, is the most common cause of ED. Diseases—such as diabetes, hypertension, kidney disease, chronic alcoholism, multiple sclerosis, atherosclerosis, vascular disease, and neurologic disease—account for about 70 percent of ED cases. Up to half of men with diabetes experience ED.

Other common medications that can cause ED include tranquilizers, appetite suppressants, and the ulcer drug cimetidine.

Testosterone is important to the erection process, and those levels begin decreasing in men after age 40. Inactivity also may play a role. Some research suggests that when the penis is relaxed for long periods of time, scar tissue forms in cells, interfering with the penis' ability to expand when filled with blood.

For years, the primary cause of erectile dysfunction had been considered to be psychological, but today most doctors and researchers think that 80 percent of cases are physical. Psychological causes might be anger at a spouse, guilt over normal but "unfaithful" sexual interest in another person, or stress and burnout at work, experts say.

Experts say that the average couple currently seeking help for the man's dysfunction has not had intercourse for two to five years.

If a pre-existing disease, medication, or other cause is determined, it should be treated before using impotence drugs. Currently, three oral drugs are available to treat ED. These drugs are not aphrodisiacs, so they have no effect on sexual desire. What they do is physically relax the blood vessels of the penis, allowing it to fill with blood and to stay erect long enough for intercourse. These medications also may cause side effects, so they aren't for everyone. Often, medication is accompanied by counseling, especially because couples seeking help for ED may have emotional issues they need to overcome, as well as the physical problems. Other treatments for ED include injection therapy, in which medication is injected into the penis; insertion of tiny pellets into the penis; bands; vacuum tubes; and surgical implants. If low testosterone is causing the ED, it can be treated with testosterone patches or injections. In rare cases, surgery may be done to improve blood flow to the penis.