Cardiovascular disease. High blood pressure, arteriosclerosis (hardening of the arteries), and other problems with the heart and blood vessels can cause impotence by hampering blood flow to the penis. The Massachusetts Male Aging study found that 15 percent of men with hypertension were completely impotent, compared with 9.6 percent of the entire study group.

Medications. The National Institutes of Health reports that many common drugs can lead to impotence, including antihistamines, antidepressants, tranquilizers, appetite suppressants, the ulcer drug cimetidine, and blood pressure medications such as beta blockers and diuretics. (If you develop impotence while taking antidepressants or blood pressure medications, however, remember that the disease itself could also be to blame.)

Diabetes. If it causes extensive damage to nerves and blood vessels, diabetes can block pleasurable sensations as well as blood flow. The NIH estimates that 35 to 50 percent of all men with diabetes suffer from impotence.

Psychological factors. According to the NIH, psychological distress lies at the root of 10 to 20 percent of all cases of impotence. If you suffer from impotence but have erections in the morning or while you sleep, there’s a good chance your problem is largely psychological. Factors that can lead to impotence include stress, fear, performance anxiety, guilt, depression, and low self-esteem. Feelings of resentment or hostility - harbored by either the person experiencing impotence or his partner - can also derail intimacy.

Prostate surgery. Not long ago, many men had to sacrifice their sex lives after undergoing surgery for prostate cancer. Today, thanks to new surgical techniques that spare the nerves and vessels feeding the penis, impotence is far from inevitable. Surgeons at Washington University School of Medicine in St. Louis recently conducted a study of cancer patients who were able to achieve erections normally before surgery. Sixty-eight percent of the men studied were still able to have erections after the prostate gland was removed. However, only 48 percent of those 70 and older regained their potency.

Smoking. Cigarettes can damage your arteries, with consequences that are enough to make even the Marlboro Man find a different habit. According to a recent report from the Centers for Disease Control, smokers between the ages of 31 and 49 are 80 percent more likely than nonsmokers of the same age to suffer from impotence.

Bicycling. Although there is controversy on this subject, a few experts say regular cycling is a major cause of impotence. A recent Boston University survey looked at more than 1,000 male cycling and running club members, with an average age of 43, and found that 4 percent of cyclists had moderate to complete impotence, compared with 1 percent of runners. There’s no doubt that pressing one’s crotch into a hard bicycle seat puts tremendous pressure on the nerves and blood vessels that feed the penis. If you ride a bike, be sure to lean back regularly to shift your weight from the groin area to the buttocks. You should also consider investing in a seat that’s specially designed to put less pressure on the groin. If you often feel numbness in your penis after bike riding, or if numbness is less frequent but lingers, schedule an appointment with your doctor or a urologist.

Other factors. Alcoholism, pelvic injuries, multiple sclerosis, and below-normal levels of testosterone are all capable of causing impotence.

How is impotence treated?

If you have impotence, schedule an appointment with your doctor or a urologist. By asking questions and running tests, a physician should be able to find the source of your problem and suggest a course of treatment. If you take prescription drugs, curing impotence may be as simple as changing your medication. Otherwise, there are various types of treatment that may help.

Sildenafil (Viagra). As you’ve undoubtedly heard, this new pill can help men achieve erections. Sildenafil, marketed under the name Viagra, works by enhancing the flow of blood to the penis. It doesn’t increase a man’s appetite for sex, and it can only cause an erection if he’s sexually aroused. In a recent study of 267 men conducted at Johns Hopkins University, 65 percent of those taking sildenafil said they were pleased with the results. The satisfaction rate was not so rosy for patients whose impotence was severe (41 percent), stemmed from nerve damage caused by diabetes (21 percent), or had resulted from prostate surgery (27 percent).

You may have heard, as well, that sildenafil can be dangerous for people with heart trouble. While there are some risks, a recent report in the American Journal of Cardiology says most heart patients can take the drug safely. The only patients who must absolutely avoid sildenafil are those who take nitrate drugs for heart trouble: Combining sildenafil with nitrates can lead to dangerously low blood pressure. In addition, men should use sildenafil with caution if they take certain hypertension medications, suffer from coronary ischemia (blockage of an artery leading to the heart), or have a combination of congestive heart failure and low blood pressure.

For heart patients who get little or no exercise, the rigors of sex may be more dangerous than sildenafil. In some cases, doctors will examine a patient’s heart during a stress test before prescribing the drug.

Other drugs. Other drugs. Men now have two more oral drugs to choose from - vardenafil (Levitra) and tadalafil (Cialis). Both work much like sildenafil, increasing blood flow to the penis to make erections possible. Other drugs used to treat impotence require that you do more than just pop a pill. Drugs such as papaverine and phentolamine can cause near-instant erections, but they have to be injected into the penis. Aside from the obvious trauma of sticking a needle into one’s penis, some men develop priapism, a painful erection that won’t subside without medical assistance. Men can also use a new needle-free device to insert a pellet of the drug alprostadil into the urethra. Erections usually occur eight to 10 minutes later and last 30 to 60 minutes. The treatment, however, can cause pain in the penis and testicles.

Testosterone. When impotence is caused by low levels of testosterone, a permanent hormone patch might be a quick cure.

Ginkgo biloba. Ginkgo leaf extract can reverse impotence caused by antidepressants in the selective serotonin reuptake inhibitor category, including fluoxetine (Prozac) or sertraline (Zoloft), according to the Natural Medicines Comprehensive Database. Some psychiatrists have also prescribed ginkgo for other types of impotence. Check with your doctor before taking it, though, because ginkgo can interact with certain drugs.

Psychotherapy. If there is no physical cause suspected, your physician may suggest counseling for both you and your partner. A psychotherapist can teach you techniques to help stay relaxed and responsive during intimate moments. Psychotherapy can also be valuable to men who are trying to overcome nonpsychological impotence.

Vacuum devices. These gadgets may look strange, but they can be helpful for some patients. The devices consist of a plastic cylinder that fits over the penis and a pump that creates a partial vacuum. The vacuum pulls more blood into the penis, causing an erection. The man removes the device and places an elastic band around the base of his penis to maintain the erection during sex.

The success of the vacuum device varies from patient to patient. A Dutch study of 67 impotent patients found that 72 percent were able to achieve erections enabling them to have sex when they took their devices home. But in an Australian study of 43 men who tried the vacuums after giving up on drug injection therapy, 81 percent eventually abandoned the procedure. Most said they quit because the machines didn’t work, so it’s possible that men who don’t respond to other treatments are less likely to benefit from vacuum devices.

Surgery. In some cases of impotence caused by injury, surgeons can cure the condition by reconstructing arteries that feed blood to the penis. This works best in younger men with minor pelvic injuries. Surgeons can also block the veins that drain the penis, although some experts question whether this is a reasonable long-term solution.

Finally, doctors can insert implants into the penis to make erections possible. The implant might be a semirigid rod that can be bent into shape or an inflatable cylinder. A man can fill the cylinder with fluid by squeezing a small pump that’s embedded in his scrotum. Thanks to recent advances, today’s implants are very effective and carry only a small risk of infections and mechanical breakdowns.

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- Chris Woolston, M.S., is a health and medical writer with a master’s degree in biology. He is a contributing editor at Consumer Health Interactive, and was the staff writer at Hippocrates, a magazine for physicians. He has also covered science issues for Time Inc. Health, WebMD, and the Chronicle of Higher Education. His reporting on occupational health earned him an award from the northern California Society of Professional Journalists.