What are the risk factors for ED?

The most common risk factors for ED are:

age over 50
diabetes
high blood pressure
high cholesterol
smoking
cardiovascular disease

These diseases over a period, can lead to a degeneration of the penile blood vessels, leading to restriction of blood flow through the arteries and to erectile tissue damage, which allows leakage of blood through the veins during erection.
Abnormally low levels of circulating testosterone may cause ED, although low testosterone is found in a minority of men who develop ED. Low levels of sexual desire, lack of energy, mood disturbances, loss of muscle strength and depression can all be symptoms of low testosterone. A simple blood test can determine if the testosterone level is abnormally low.  Low levels of testosterone can be replaced by using a number of different delivery systems (e.g., shots, skin patches, gels, sub-dermal implants).
The choices made in life can lead to degeneration of the erectile tissue and the development of ED. Smoking, drug or alcohol abuse, particularly over periods of time, will compromise the blood vessels of the penis. Lack of exercise and a sedentary lifestyle also contribute to the development of ED. Modifying these risk factors may contribute to overall health and in some individuals correct mild ED.
Patients undergoing surgery or radiation therapy for cancer of the prostate, bladder, colon or rectum are at high risk for the development of ED. Drugs used to treat these risk factors listed above may also lead to or worsen ED.
Another cause of ED is peripheral neuropathy in which the nerves leading to the penis fail to send coordinated signals to the penis. Peripheral neuropathy can be caused by diabetes, HIV infection, certain medications and other less common conditions.