Causes and Classification of Erectile Dysfunction
Normal erectile function requires the coordination of psychological, hormonal, neurological, vascular, and cavernosal factors. Alteration in any one of these factors is sufficient to cause erectile dysfunction. Not uncommonly, a combination of factors is involved.

Psychogenic Erectile Dysfunction
Most causes of erectile dysfunction were once considered to be psychogenic, but current evidence suggests that up to 80 percent of cases have an organic cause. Regardless of the primary etiology, a psychologic component frequently coexists.

Psychogenic influences are the most likely causes of intermittent erectile failure in young men.

Anxiety about "performance" may result in inhibitory sympathetic nervous system activity, and anticipatory anxiety can make the condition self perpetuating. Common causes of psychogenic erectile dysfunction include performance anxiety, a strained relationship, lack of sexual arousability, and overt psychiatric disorders such as depression and schizophrenia.
A psychogenic component is often present in older men, secondary to an organic cause.

Neurogenic Erectile Dysfunction
Neurologic disorders such as Parkinson's disease, Alzheimer's disease, stroke, and cerebral trauma often cause erectile dysfunction by decreasing libido or preventing the initiation of an erection. In men with spinal cord injuries, the degree of erectile function depends largely on the nature, location, and extent of the lesion. Sensory involvement of the genitalia is essential to achieve and maintain reflexogenic erection, and this becomes more important as the effect of psychological stimuli abates with age.

Hormonal Causes of Erectile Dysfunction
Androgen deficiency decreases nocturnal erections and libido. However, erection in response to visual sexual stimulation is preserved in men with hypogonadism, demonstrating that androgen is not essential for erection. Hyperprolactinemia from any cause results in both reproductive and sexual dysfunction because prolactin inhibits central dopaminergic activity and therefore the secretion of gonadotropin-releasing hormone, resulting in hypogonadotropic hypogonadism.

Vascular Causes of Erectile Dysfunction
Common risk factors associated with generalized penile arterial insufficiency include hypertension, hyperlipidemia, cigarette smoking, diabetes mellitus, and pelvic irradiation. Focal stenosis of the common penile artery most often occurs in men who have sustained blunt pelvic or perineal trauma (e.g., from bicycling accidents). In men with hypertension, erectile function is impaired not by the increased blood pressure itself but by the associated arterial stenotic lesions.

Failure of the veins to close during an erection (veno-occlusive dysfunction) can cause erectile dysfunction. Veno-occlusive dysfunction may be caused by the formation of large venous channels draining the corpora cavernosa, degenerative changes to the tunica albuginea (due to Peyronie's disease, old age, or diabetes mellitus) or traumatic injury (penile fracture), structural alterations of the cavernous smooth muscle and endothelium, poor relaxation of trabecular smooth muscle (in anxious men with excessive adrenergic tone), and shunts acquired as a result of operative correction of priapism.

Erectile Dysfunction Due to Other Systemic Diseases and Aging
Sexual function progressively declines in healthy aging men. For example, the latent period between sexual stimulation and erection increases, erections are less turgid, ejaculation is less forceful, the ejaculatory volume decreases, and the refractory period between erections lengthens. There is also a decrease in penile sensitivity to tactile stimulation, a decrease in the serum testosterone concentrations, and an increase in cavernous muscle tone.

Patients with diabetes mellitus have high rates of erectile dysfunction as a result of vascular disease and autonomic dysfunction. About 60-65 percent of men with chronic diabetes mellitus have erectile dysfunction. In addition to affecting small vessels, diabetes may affect the cavernous nerve terminals and endothelial cells, resulting in a deficiency of neurotransmitters.

Chronic renal failure has frequently been associated with diminished erectile
function, impaired libido, and infertility. The mechanism is probably multifactorial, involving low serum testosterone concentrations, vascular insufficiency, use of multiple medications, autonomic and somatic neuropathy, and psychological stress. Men with angina, myocardial infarction, or heart failure may have erectile dysfunction due to anxiety, depression, or concomitant penile arterial insufficiency.

Drug-Induced Erectile Dysfunction
In general, drugs that interfere with central neuroendocrine or local neurovascular control of penile smooth muscle have the potential for causing erectile dysfunction. Although we do now know about the mechanisms behind the causation of the erectile dysfunction, in many cases, the mechanism is still tp ne precisely elucidiated.

The drugs may have a direct effect or may act indirectly through bringing about nerve, vascular or hormonal imbalance.

Medications used to treat hypertension, depression and other psychiatric disorders are most commonly associated with erectile dysfunction. Central neurotransmitter pathways, including serotonergic, noradrenergic, and dopaminergic pathways involved in sexual function, may be disturbed by antipsychotic, antidepressant, and centrally acting antihypertensive drugs.

ß-Adrenergic–blocking drugs may cause erectile dysfunction by potentiating æ₁-adrenergic activity in the penis. Thiazide diuretics have been reported to produce erectile dysfunction, but the cause is unknown. Spironolactone can cause erectile failure as well as gynecomastia and a decrease in libido.

Cigarette smoking may induce vasoconstriction and penile venous leakage because of its contractile effect on the cavernous smooth muscle. Alcohol in small amounts improves erection and increases libido because of its vasodilatory effect and the suppression of anxiety; however, large amounts can cause central sedation, decreased libido, and transient erectile dysfunction. Chronic alcoholism may cause hypogonadism and polyneuropathy, which may affect penile nerve function. Cimetidine, a histamine H₂-receptor antagonist, has been reported to decrease libido and cause erectile failure; it acts as an antiandrogen and can cause hyperprolactinemia. Other drugs known to cause erectile dysfunction are estrogens and drugs with antiandrogenic action, such as ketoconazole and cyproterone acetate.

As many as 25 percent of cases of erectile dysfunction are related to medication side effects.