The mechanisms for erection are fairly complex: A sensory stimulus triggers the brain to send nerve impulses down through the spinal cord. These signals trigger the release of a chemical messenger that causes the vessels supplying blood to the penis to dilate. The rod-shaped spongy tissues (corpora cavernosa) in the penis then fill with blood and expand, pressing against the veins that normally allow blood to drain from the penis, thus producing an erection. Interference with any part of this process - whether psychological or physiological - may cause erectile dysfunction.
 
Although an occasional inability to achieve or maintain an erection is common and not a sign of a chronic problem, a doctor should be consulted if erectile dysfunction persists. Treatment depends upon the underlying cause of erectile dysfunction.

• Emotional and psychological difficulties such as guilt or anxiety (especially performance anxiety, in which fear of not having an erection becomes so distracting that it becomes a self-fulfilling prophecy).
• Conditions that affect the brain and decrease the libido (sex drive), including depression or schizophrenia; use of drugs that act on the central nervous system (sedatives, antidepressants, some antihypertensive, antipsychotic, and alcohol); and chronic illnesses such as heart, lung, kidney, or liver disease, and certain types of cancer.
• Hormonal disturbances that decrease the libido, including diminished testosterone levels, elevated prolactin (due to a pituitary tumor) and hyper- or hypothyroidism.
• Brain disorders (that do not affect libido but have neurological consequences that affect sexual functioning), including brain tumor and stroke.
• Spinal cord disorders, such as multiple sclerosis or spinal cord trauma.
• Damage to the peripheral nerves due to diabetes mellitus or pelvic surgery for disorders such as prostate cancer or rectal cancer.
• Medications that can sometimes cause sexual dysfunction, including anticholinergics, antihistamines, beta-blockers (and other kinds of antihypertensives), and long-term use of nicotine.
• Peripheral vascular disease (impaired blood flow to the extremities and the penis).
• Fatigue.
• Advancing age.

Inability to achieve or maintain an erection sufficient for satisfactory sexual performance.

• Avoid nicotine, alcohol, and other drugs.
• Your doctor may change your prescriptions if a medication has caused erectile dysfunction.
• Psychological counseling may be recommended when erectile dysfunction has an emotional or psychological cause.
• Testosterone injections or skin patches are given if blood testosterone levels are low.
• Hyperthyroidism or hypothyroidism is treated if necessary.
• Bromocriptine therapy is given to correct elevated prolactin levels.
• Three oral medications are currently available for treating erectile dysfunction: Viagra (sildenafil), Levitra (vardenafil), and Cialis (tadalafil). All three drugs treat erectile dysfunction by permitting blood to flow into the penis during sexual stimulation - the drugs do not produce erections in the absence of sexual stimulation. Viagra and Levitra improve erections for about four hours; Cialis is effective for 24 to 36 hours. Side effects of the three drugs are slightly different. The drugs are not recommended for men who take nitrate-containing medication (such as nitroglycerin) or men who have recently had a stroke, heart attack, or life-threatening arrhythmia.
• A special vacuum device may be used to produce an erection. Air is pumped out of a plastic tube placed securely over the penis, and the resulting vacuum pulls blood into the corpora cavernosa within a few minutes. The device is removed and a rubber band is placed at the base of the penis to maintain the erection.
• Self-administered injections of alprostadil, a vasodilator drug, dilate the blood vessels in the penis to produce an erection. Your doctor will instruct you on the correct injection technique.
• Surgical implants for the penis are available. In one procedure, an inflatable device with a small fluid reservoir is inserted. In another, flexible rods are implanted that can be either bent upward to produce an erection or tucked close to the body.
• In rare cases of erectile dysfunction, your doctor may advise vascular surgery to improve blood flow to the penis.

• Have no more than two alcoholic beverages a day.
• Don't smoke.
• Talk to a therapist about improving communication with your sexual partner.