Incidence of Erectile Dysfunction
Half the male population above the age of 40 years is known to suffer from erectile dysfunction. However, the severity of the disorder varies from individual to individual. Some fail to have an erection, whereas others cannot get a hard erection, whilst others may be unable to maintain this hard erection for a sustained period of time.  The incidence is also known to increase with age, with the occurrence of complete erectile dysfunction increasing from 5% at 40 years of age, to 15% above the age of 70. To understand this condition better, it is imperative to examine its causes first.

Causes of Erectile Dysfunction
There are a large number of causes for the development of erectile dysfunction in men. A multitude of factors may be simultaneously present in a significant number of people. Some of the major causes for the occurrence of erectile dysfunction are as follows:

Old age
With the advancement of age, structural changes occur in the muscles and tissues contained within the penis, and this may contribute to the inability to achieve or maintain an erection. Elderly people are also more prone to suffering from systemic diseases such as hypertension, diabetes mellitus, stroke etc. which may all be independent risk factors for the development of erectile dysfunction.

High blood pressure (or hypertension)
It may be detrimental to the ability to achieve an erection, either via decreasing the amount of nitric oxide released from the nerves around the arteries or via the clogging of the arteries inside the penis with plaques (atherosclerosis). This interferes with the normal functioning of the arteries required during erection.

Diabetes mellitus
Diabetes is known to affect the development of erectile dysfunction in a variety of ways. Diabetics have been shown to develop erectile dysfunction 10-15 years earlier than their counterparts. In this disease, hardening of the arteries (or atherosclerosis) is known to occur at an earlier age and with greater severity. Coupled with this action in the arteries, diabetes also damages the nerves and muscles of the penis, thereby impeding the ability to achieve an erection amongst males. Other risk factors that may cause the development of erectile dysfunction amongst diabetic males are a long duration of illness, poor glucose control, obesity, and smoking.

Heart Disease
Many physicians now suggest screening new onset erectile dysfunction for silent coronary artery disease. This is because atherosclerosis (a disease affecting the arteries of the body) is an independent risk factor for the development of erectile dysfunction. Atherosclerosis causes the walls of the arteries to become hardened with plaques, thereby impeding blood flow to the penis. Some studies have shown a direct correlation between the severity of coronary artery disease and erectile dysfunction.

Smoking
Cigarette smoking is another independent causative factor responsible for the development of erectile dysfunction. Nicotine found in smoke may increase atherosclerosis in the arteries, whilst also contribute to vasospasm (a condition where the arteries become constricted and blood flow to the organ are reduced).

Substance Abuse
Narcotics, alcohol abuse, heroin, cocaine, marijuana, amphetamines and crystal meth are known to be implicated in erectile dysfunction. Alcoholism, apart from damaging the nerves around the penis, also causes low testosterone levels and shrinkage (atrophy) of the testes.

Brain or spinal cord damage
These nervous disorders may occur due to trauma during accidents, injuries during operations (such as damage to the pelvic nerves in prostate or colon surgery), diabetes mellitus, multiple sclerosis, chemoradiotherapy for cancers etc.

Psychological factors
Many psychological disorders are known to be solely responsible for the development of erectile dysfunction. In fact, the medications used to treat these disorders may also contribute to its development. Some of the notable diseases are anxiety, depression, increased stress levels, performance anxiety, feelings of guilt, low self-esteem and post-traumatic stress disorder.

Medications
A large number of medicines may also be causative for this disorder. These include anti-hypertensives, anti-histaminics, anti-depressants, anti-psychotics etc. Some of the commonly implicated drugs include beta blockers, digoxin, thiazides, amitriptyline, metoclopramide, cimetidine, phenytoin, lithium, calcium channel blockers, amphetamines, and indomethacin.

Decreased testosterone levels
Testosterone is the primary male sex hormone and performs a wide range of functions. It is responsible for maintaining the sex drive (or libido), whilst also being necessary for keeping nitric oxide levels high inside the penis. As such, diseases leading to low testosterone levels may present with erectile dysfunction as one of their main features.

Others
Amongst the other notable risk factors involved in the development of erectile dysfunction are high cholesterol levels, surgeries in the pelvic region, trauma to the pelvis and perineum structures, radiotherapy, endocrinological and other medical disorders.