Erection Problems (Erectile Dysfunction)

Topic Overview

What are erection problems?

A man has erection problems if he cannot get or keep an erection that is firm enough for him to have sex. Erection problems are also called erectile dysfunction or impotence.

Erection problems can occur at any age. But they are more common in older men, who often have other health problems. Treatment can help both older and younger men.

What causes erection problems?

Erection problems may be caused by physical problems, such as injury to nerves or loss of blood supply to the penis.

They can also be linked to other health problems. These include diabetes, high blood pressure, high cholesterol, and atherosclerosis. Erection problems can also be linked to problems with the nervous system, such as multiple sclerosis and Parkinson's disease.

Many medicines for other health problems may cause erection problems, but most do not. If you recently began taking a new medicine and started having erection problems, this could be a side effect of the medicine. Talk with your doctor. He or she may be able to change the dose or type of medicine you take.

Men who drink too much alcohol, smoke, or use illegal drugs also are at risk for erection problems.

Anxiety, stress, or depression can cause erection problems.

Other causes include surgery, such as for prostate cancer, or injury to the pelvic area.

What are the symptoms?

The only symptom of an erection problem is being unable to get and keep an erection that is firm enough to have sex. But even with an erection problem, a man may still have sexual desire and be able to have an orgasm and to ejaculate.

How are erection problems diagnosed?

Your doctor can find out if you have an erection problem by asking questions about your health and doing a physical exam. Your doctor will want to know if the problem happens all the time or just from time to time. The exam, lab tests, and sometimes mental health tests can help find out the cause of the problem.

How are they treated?

There are a number of treatments for erection problems. Doctors usually start with lifestyle changes and medicines. They usually don't advise surgery or other treatments unless those first steps don't help.

Treatment can include:

• Making lifestyle changes, such as avoiding tobacco, drugs, and alcohol. It may also help to talk about the issue with your partner, do sensual exercises, and get counseling.
• Finding and then stopping medicines that may be causing the problem. In some cases you can take a different medicine that does not cause erection problems.
• Taking prescription medicine that can help you get erections. These include pills such as sildenafil (Viagra), tadalafil (Cialis), and vardenafil (Levitra). Check with your doctor to see if it is safe for you to take one of these medicines with your other medicines. These can be dangerous if you have heart disease that requires you to take nitroglycerin or other medicines that contain nitrates.
• Taking medicines and getting counseling for depression or anxiety.
• Using vacuum devices or getting shots of medicine into the penis.
• Having surgery to place an implant in the penis.

Can you prevent erection problems?

Because erection problems are most often caused by a physical problem, it’s important to eat healthy foods and get enough exercise to help you stay in good health.

To reduce your risk of having an erection problem, do not smoke, drink too much alcohol, or use illegal drugs.

You may be able to avoid erection problems related to anxiety and stress by talking with your partner about your concerns. This may help you relax.

Frequently Asked Questions

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Cause

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.

A vasectomy usually does not cause an erection problem. But pain after the operation may affect sexual performance for a time, and if a man was not comfortable with his decision to have a vasectomy, or is having second thoughts, it could affect him psychologically.

Psychological causes of erection problems include depression (which also has a physical component), anxiety, stress, grief, or problems with current or past relationships. These interfere with the erection process by distracting the man from things that would normally arouse him. Erection problems in men younger than 40 who have no physical risk factors are more likely to be caused by psychological factors than physical causes.

• Relationship problems can lead to erection problems. This may happen when a man has been widowed or loses sexual interest in a particular partner.
• Some men develop erection problems when they contemplate marriage.
• Some men may have difficulty having sexual intercourse with their partner after their first child is born.

Symptoms

Symptoms of erection problems (erectile dysfunction) include being unable to:

• Have an erection at any time, either with masturbation or with a sex partner.
• Maintain an erection firm enough for sexual intercourse.
• Maintain an erection long enough to complete sexual intercourse.

Even with an erection problem, a man may still have sexual desire and be able to have an orgasm and to ejaculate.

What Happens

Most men have occasional erection problems (erectile dysfunction). But when erection problems become persistent, they can affect your self-image, sex life, and relationship. When you have persistent erection problems, "performance anxiety" can worsen the problem. If you cannot maintain an erection that is firm enough for intercourse, or you have an orgasm before or immediately after entering your partner (premature ejaculation), you may feel frustrated and believe you are not pleasing your partner. All of these factors could influence how you view your relationship.

Fortunately, many of the physical and psychological factors that cause erection problems respond to treatment.

What Increases Your Risk

Your risk of developing an erection problem (erectile dysfunction) increases with age.^{1, 2}

• About half of the men between ages 40 and 70 report erection problems.
• More than 15 out of 100 men in their 40s report having a moderate erection problem. But twice as many (about 30 out of 100) men in their 70s say they have a moderate erection problem.
• While only about 5 out of 100 men in their 40s say their erection problems are severe, three times as many (15 out of 100) men in their 70s report severe problems.

Diseases, physical or psychological problems, and certain activities also may increase your risk.

Diseases that affect blood vessels include:

• Diabetes. About half of men with diabetes develop erection problems.²
• High blood pressure or other blood vessel diseases.
• High cholesterol or low HDL (high-density lipoprotein) cholesterol.

Diseases or procedures that affect nerves include:

• Diabetes.
• Multiple sclerosis.
• Stroke.
• Parkinson's disease.
• Having had surgery involving the prostate, bladder, rectum, or urethra. These procedures may cause injury to nerves involved in achieving and maintaining an erection.

Other conditions include:

• Thyroid problems.
• Low levels of the hormones needed for the normal development and function of the sex organs (hypogonadism), leading to low testosterone levels.

Injuries or treatment include:

• Injury to the penis or pelvic region.
• Injury to the spinal cord or nerves to the penis.
• Pelvic surgery.
• Radiation in the pelvic area.

Medicines and other substances that increase your risk include:

• Some medicines to treat high blood pressure or depression.
• Long-term (chronic) alcohol abuse.
• Drug abuse.
• Tobacco use.

Psychological risk factors include:

• Depression.
• Anxiety or stress.
• Relationship problems.
• A recent major life change (birth of a child, retirement, job change, loss or death of a partner, divorce, or marriage).

Activities that constrict blood flow to the penis—such as frequent long-distance bicycle riding on a hard, narrow saddle—may increase a man's risk of developing an erection problem. But experts continue to debate this issue.

A vasectomy usually does not cause erection problems. But pain after the operation may affect sexual performance for a time, and if a man was not comfortable with his decision to have a vasectomy, or is having second thoughts, it could affect him psychologically.

When To Call a Doctor

Seek care immediately if your erection lasts longer than 3 hours.

Seek care immediately if you have taken phosphodiesterase-5 inhibitors, such as sildenafil (Viagra), tadalafil (Cialis), or vardenafil (Levitra), in the past 24 hours and are having chest pain. Tell your health professional you are taking this medicine. Do not use any form of nitroglycerin if you have chest pain and have taken sildenafil (Viagra) or vardenafil (Levitra) in the past 24 hours, or tadalafil (Cialis) in the past 48 hours.

Call a health professional if erection problems occur with:

• Any type of injury to the back, legs, buttocks, groin, penis, or testicles.
• A loss of pubic or armpit hair and an enlargement of the breasts.

Make an appointment to see a health professional within 1 to 2 weeks if an erection problem occurs more than 25% of the time and the problem:

• Occurs with a persistent backache.
• Occurs after you start taking a new medicine or change the dose of a medicine.
• Is affecting your self-image or sense of well-being.
• Has not improved despite self-care.

If your erection problem is occasional, there is no reason to call your health professional. If it occurs frequently but does not bother you or your partner, you may or may not choose to call your doctor.

Watchful Waiting

Watchful waiting means a "wait-and-see" approach. A single episode of an erection problem is often a temporary and easily reversible problem. Do not assume it will happen again. If possible, forget about it and expect a more successful experience the next time. If you or your partner is concerned about it, talk about the problem and openly discuss your fears and anxieties.

If self-care has not helped after 2 weeks and you are concerned about your inability to have an erection, see a health professional who has experience in dealing with erection problems.

Who To See

Some health professionals, including doctors and mental health professionals, may not feel comfortable discussing sexuality and erection problems. Ask your health professional if he or she feels comfortable with and has experience in working with men who have erection problems.

The following health professionals can evaluate symptoms of erection problems:

• Family medicine doctor
• Internist
• Nurse practitioner
• Physician assistant
• Urologist

If it is possible that a psychological problem is contributing to your erection problem, your doctor may refer you to a health professional such as a:

• Psychiatrist.
• Psychologist.
• Certified licensed social worker.
• Counselor with special training in sexuality or relationship problems.

To prepare for your appointment, see the topic Making the Most of Your Appointment.

Exams and Tests

Because both physical and psychological factors are often involved, it may be difficult to diagnose the exact cause of your erection problem.

As part of the initial evaluation, your health professional may:

• Review your risk factors for an erection problem. For more information, see the What Increases Your Risk section of this topic.
• Ask questions about your sexual function.
• Do a complete physical examination of the abdomen, penis, prostate, rectum, and testicles.
• Do laboratory tests for:
  • Testosterone. A low testosterone level may reduce sexual desire, leading to an erection problem.
  • Prolactin. A high level of prolactin (hyperprolactinemia) may indicate a small tumor in the brain, which can affect other hormone levels in the body. When hormone levels are affected, an erection problem may result.
  • Thyroid hormone. Either a high or low level of thyroid hormone may cause problems with sexual desire or erections.
  • Blood glucose. Elevated blood sugar (glucose) levels may indicate the presence of diabetes, which can contribute to erection problems.

At this point, oral medicines such as sildenafil (Viagra), tadalafil (Cialis), and vardenafil (Levitra) are often recommended unless an easily treated cause (such as a medicine side effect or testosterone deficiency) has been identified. If oral medicines do not work or your doctor feels more testing is necessary, he or she may recommend:

• Specific tests for erection problems, such as a nocturnal penile tumescence testing (rigidity test) and an intracavernosal injection test. These both help determine whether physical or psychological factors are causing your erection problem.
• A psychological evaluation, if a psychological issue is suspected.

An ultrasound may be done if your doctor suspects you have a circulation problem (peripheral arterial disease). Even if the blood vessels cannot be repaired, it may be helpful to know if you have this condition because it may mean you have a risk for other blood vessel (vascular) problems. In very rare instances, you may have angiography—an X-ray test of blood flow through an artery. Its usefulness is limited because treatment to repair the blood vessels that supply blood to the penis is appropriate for very few men.

Treatment Overview

Treatment for an erection problem (erectile dysfunction) depends on the cause of the problem, which may be psychological, physical, or a combination of both. Erection problems that have one or more major physical causes also often have psychological factors that make the problem worse and make treatment more complicated.

Many doctors take a stepwise approach to treating erection problems, using the least invasive treatments first. These steps are:

1. Discovering and then eliminating medications that may be causing your condition. In some cases a different medicine can be tried.
2. Trying an oral medicine. Medicines used for erection problems include phosphodiesterase-5 inhibitors such as sildenafil (Viagra), tadalafil (Cialis), and vardenafil (Levitra). One of these medicines may be tried unless an easily treated cause—such as a medicine side effect or testosterone deficiency—has been identified.
3. Getting counseling if a psychological cause is suspected.
4. Trying medicines that are injected or inserted into the penis.
5. Trying a vacuum device.
6. Trying penile implant surgery.

Counseling (also called psychotherapy) or behavioral therapy may be appropriate even if your erection problem has a physical cause. It may be offered if your health professional suspects psychological issues play a role in your erection problems.

For more information on treatment options, see:

Should I use a phosphodiesterase-5 inhibitor (such as sildenafil [Viagra], tadalafil [Cialis], or vardenafil [Levitra]) for erection problems?

Should I use injections for erection problems?

What To Think About

It is important to involve your partner in your decision regardless of the treatment you choose.

Oral medicines have revolutionized the treatment of erection problems, and they are commonly tried first before other medicine or surgery.

Although phosphodiesterase-5 inhibitors have relatively few side effects, they can be dangerous in certain men. If you are taking nitrate-containing medications, such as nitroglycerin, you cannot use sildenafil (Viagra), tadalafil (Cialis), or vardenafil (Levitra). You also should not take certain alpha-blockers—used to lower blood pressure and to treat an enlarged prostate gland—with these medicines because of the risk of a dangerous drop in blood pressure. Check with your health professional to see whether you can take sildenafil (Viagra), tadalafil (Cialis), or vardenafil (Levitra) with your alpha-blocker.

Many men overestimate how important being able to have erections is to their relationships. Some men find that once they are able to have erections again, the hassle of using the treatment is not worth the effort. Other men may find that being able to have erections doesn't change their relationship as much as they or their partners had expected.

Prevention

You may be able to avoid erection problems (erectile dysfunction) related to anxiety and stress by taking a more relaxed approach to lovemaking. Talk to your partner about your problems and concerns. Sexual intimacy is a form of communication. If you and your partner talk about your lovemaking, it will help reduce your stress and anxiety, and you may become more relaxed.

Erections may gradually become more difficult to get and maintain as you get older. But foreplay—erotic stimulation before intercourse—and the right environment can help increase your ability to have an erection, regardless of your age.

Other things you can do that may reduce your risk for developing an erection problem include:

• Quitting smoking.
• Avoiding use of alcohol and illegal drugs.
• Keeping your cholesterol level low to reduce the risk of hardening of the arteries (atherosclerosis).
• Treating high blood pressure.
• Keeping your blood sugar in a safe range if you have diabetes.
• Exercising regularly.
• Losing weight, especially if you are obese.³
• Lowering stress in your life.

Home Treatment

In some cases you can treat occasional episodes of erection problems (erectile dysfunction) at home, without a health professional's help. But do involve your partner in the process, and don't be embarrassed about seeking professional help if erection problems are consistent and troublesome. You may be able to help yourself by:

• Changing lifestyle factors that may affect erections. These factors include alcohol, smoking, and using illegal drugs. Losing weight and exercising more has also been helpful for some men.
• Examining your relationships and life events.
• Talking with your partner.
• Trying sensual exercises with your partner.

Some men may try methods available in health food stores or through magazine advertisements. Most of these methods have never been medically proven to work, may be unsafe, and are often expensive. They are not recommended.

Medications

Medicines that can help produce an erection may be used to treat erection problems (erectile dysfunction) that are caused by blood vessel (vascular), hormonal, nervous system, or psychological problems. They also may be used along with counseling to treat erection problems that have psychological causes.

If erection problems could be caused by a prescribed medicine, it may be possible to change the dose or try another medicine. Do not change or stop taking any medicine without first talking with your health professional.

Medication Choices

Commonly used oral medicines include:

• Phosphodiesterase-5 inhibitors (PDE-5 inhibitors) such as sildenafil (Viagra), tadalafil (Cialis), and vardenafil (Levitra).

Other medicines that may be used include:

• Injected medications.
• Intraurethral alprostadil (MUSE).

Hormones and other medicines may be prescribed for men who have low testosterone or high prolactin levels.

For more information on medication choices, see:

Should I use a phosphodiesterase-5 inhibitor (such as sildenafil [Viagra], tadalafil [Cialis], or vardenafil [Levitra]) for erection problems?

Should I use injections for erection problems?

What To Think About

PDE-5 inhibitors and heart problems

Health experts have debated the use of PDE-5 inhibitors in men with heart disease, because deaths have been reported after use of sildenafil (Viagra). The 1999 American College of Cardiology/American Heart Association (ACC/AHA) Expert Consensus Document noted that PDE-5 inhibitors may be dangerous for people who:⁴

• Have coronary artery disease (CAD).
• Have heart failure and low blood pressure.
• Are taking many different drugs for high blood pressure.

But several more recent studies have reported that some men with heart problems may be able to take PDE-5 inhibitors safely. Talk to your health professional about whether PDE-5 inhibitors are appropriate for you.

• One study found no evidence of increased risk for heart attack or CAD in men who use sildenafil (Viagra).⁵
• Another study in men with CAD reported that sildenafil (Viagra) does not lead to heart attack and that heart attacks and other cardiovascular problems reported after taking Viagra may be related more to the physical activity of intercourse than to the medicine.⁶
• A study of 35 men found that Viagra is safe for men with moderate heart failure.⁷

Sexual activity is exercise. If you have a heart condition and have not been sexually active for a while, talk with your health professional to make sure you can engage in sexual activity safely.

PDE-5 inhibitors should never be used if you may need to take a nitrate-containing medication, such as nitroglycerin. Taking nitroglycerin and a PDE-5 inhibitor within 24 hours of each other may greatly lower your blood pressure, which could lead to a heart attack, stroke, or death.

If you are taking a PDE-5 inhibitor and are going to have a test for heart disease, be certain your doctor knows you are taking it. You should not take sildenafil (Viagra) or vardenafil (Levitra) for 24 hours before the test. Do not take tadalafil (Cialis) for at least 48 hours before the test. Then if you develop a problem during the test, it will be safe to use nitrate-containing medicines such as nitroglycerin.

Although oral medicines for erection problems can be purchased over the Internet, you should still talk with your health professional before using this medicine. This is especially important if you have a heart problem.

If you are using a combination of drugs for high blood pressure, PDE-5 inhibitors could result in low blood pressure (hypotension). Also for this reason, you should not take alpha blockers—used to lower blood pressure and to treat an enlarged prostate—with these medicines without talking to your health professional; the combination could cause a dangerous drop in blood pressure.

Surgery

In a few cases, surgery may be an option to treat erection problems (erectile dysfunction). Surgery will rarely be recommended before nonsurgical treatment and counseling have been tried.

Surgery to repair the blood vessels of the penis may be an option for a small number of men younger than 40 who had an injury that caused poor blood flow to the penis. Surgery to repair blood vessels should only be done at a teaching hospital that has experts in this type of surgery.

Surgery on the pelvic area, such as prostate or bladder surgery, can sometimes damage nerves there and cause erection problems. But this is much less common than it used to be. For example, a surgeon now may do a nerve-sparing radical prostatectomy as treatment for prostate cancer. Some surgeons do nerve grafts as part of surgery for prostate cancer when the nerves cannot be saved. In a nerve graft surgery, the surgeon takes nerves from another part of your body and uses them to replace nerves damaged during surgery.

Surgery Choices

Penile implants

Surgery on penile blood vessels

What To Think About

Give careful consideration to nonsurgical options and to the possible risks of surgery. Include your partner in your decision.

Doctors who specialize in conditions of the urinary tract (urologists) perform most penile implants. Specially trained urologic surgeons usually do blood vessel repair surgery.

Other Treatment

Other treatments for erection problems (erectile dysfunction) include vacuum devices and counseling (psychotherapy).

Other Treatment Choices

• Vacuum devices are useful for all types of erection problems—physical, psychological, or both. The device has a tube you place around the penis. You pump the device to create a vacuum that leads to an erection. Then you place a band around the base of the penis to maintain the erection for up to 30 minutes.
• Counseling (psychotherapy) is recommended for men whose erection problems are caused, at least in part, by psychological factors. Psychological factors seem to be involved in many cases of erection problems. Treatment may include sex therapy, which focuses on methods to improve attitudes toward sex and specific sexual techniques. Counseling also may be used with medication treatment or vacuum devices for erection problems that have psychological and physical causes. Evidence shows that group therapy helps with erection problems in some men. Adding group therapy to treatment with sildenafil (Viagra) helped more than Viagra alone. Men who were taking part in group therapy also were more likely to keep taking their medicine.⁸

Although medicines are usually the main treatment for erection problems, some men try complementary therapies. If you don't want to use medicine, or medicine doesn't work for you, you may want to talk with your doctor about some of the following options. Most of these treatments need more research before doctors can know if they work for sure.⁹

• Acupuncture may work for some men with erection problems.
• Ginseng has been shown to work for some men with erection problems. But because it is sold as a dietary supplement, it is hard to know if you are getting the right amount.
• Some men take the amino acid L-arginine as a dietary supplement to try to treat erection problems. The amino acid increases the amount of nitric oxide in the blood, which relaxes blood vessels. In theory, L-arginine could improve erections. But L-arginine may be harmful.¹⁰
• Although the antidepressant medicine trazodone has been used for treating erection problems, there is no current evidence that it is effective for this purpose.¹
• Some men who have low zinc levels in their body have had success using zinc supplements to treat erection problems. But high doses of zinc can be dangerous.
• Some men have tried yohimbine to treat erection problems, but it is rarely used.

What To Think About

No matter what approach you use to treat an erection problem, including your partner in the decision is helpful and may improve results.

Other Places To Get Help

Online Resource

Organizations

Related Information

• Premature Ejaculation

References

Citations

1. Tharyan P, Gopalakrishanan G (2006). Erectile dysfunction, search date August 2006. Online version of BMJ Clinical Evidence. Also available online: http://www.clinicalevidence.com.
2. Bella AJ, Lue TF (2008). Male sexual dysfunction. In EA Tanagho, JW McAninch, eds., Smith's General Urology, 17th ed., pp. 589–610. New York: McGraw-Hill.
3. Esposito K, et al. (2004). Effect of lifestyle changes on erectile dysfunction in obese men. JAMA, 291(24): 2978–2984.
4. Cheitlin MD, et al. (1999). Use of sildenafil (Viagra) in patients with cardiovascular disease. ACC/AHA expert consensus document. Circulation, 99(1): 168–177.
5. Shakir SAW, et al. (2001). Cardiovascular events in users of sildenafil: Results from first phase of prescription monitoring in England. BMJ, 322(7287): 651–652.
6. Arruda-Olson AM, et al. (2002). Cardiovascular effects of sildenafil during exercise in men with known or probable coronary artery disease. JAMA, 287(6): 719–725.
7. Webster LJ, et al. (2004). Use of sildenafil for safe improvement of erectile function and quality of life in men with New York Heart Association classes II and III congestive heart failure. Archives of Internal Medicine, 164(5): 514–520.
8. Melnik T, et al. (2007). Psychosocial interventions for erectile dysfunction. Cochrane Database of Systematic Reviews (3).
9. Lue TF, Broderick GA (2007). Evaluation and nonsurgical management of erectile dysfunction and premature ejaculation. In AJ Wein, ed., Campbell-Walsh Urology, 9th ed., vol. 1, pp. 750–787. Philadelphia: Saunders Elsevier.
10. Schulman SP, et al. (2006). L-arginine therapy in acute myocardial infarction. JAMA, 295(1): 58–64.

Other Works Consulted

• American Urological Association (2005). Management of Erectile Dysfunction: An Update. Baltimore: American Urological Association. Also available online: http://www.auanet.org/timssnet/products/guidelines/main_reports/edmgmt/content.pdf.
• Derby CA, et al. (2000). Modifiable risk factors and erectile dysfunction: Can lifestyle changes modify risk? Urology, 56(2): 302–306.
• Kloner RA, et al. (2001). Effect of sildenafil in patients with erectile dysfunction taking antihypertensive therapy. American Journal of Hypertension, 14(1): 70–73.
• Lewis RW, Munarriz R (2007). Vascular surgery for erectile dysfunction. In AJ Wein, ed., Campbell-Walsh Urology, 9th ed., vol. 1, pp. 802–817. Philadelphia: Saunders Elsevier.

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