- Obtaining full erections at some times, such as when asleep (when the mind and psychological issues, if any, are less present), tends to suggest the physical structures are functionally working. However, the opposite case, a ''lack'' of nocturnal erections, does not imply the opposite, since a significant proportion of sexually functional men do not routinely get nocturnal erections or wet dreams.
- Obtaining erections which are either not rigid or full (''lazy erection''), or are lost more rapidly than would be expected (often before or during penetration), can be a sign of a failure of the mechanism which keeps blood held in the penis, and may signify an underlying clinical condition, often cardiovascular in origin....
- Other factors leading to erectile dysfunction are diabetes mellitus (causing neuropathy) or hypogonadism (decreased testosterone levels due to disease affecting the testicles or the pituitary gland).
- Hormonal Disorders (pituitary gland tumor; low or abnormally high levels of the hormone testosterone).
- Arterial Disorders (peripheral vascular disease, hypertension; reduced blood flow to the penis).
- Venous leak
- Cavernosal Disorders (Peyronie's disease.)
- Psychological causes: stress, mental disorders (clinical depression, schizophrenia, substance abuse, panic disorder, generalized anxiety disorder, personality disorders or traits.), psychological problems, negative feelings.
- Surgery (radiation therapy, surgery of the colon, prostate, bladder, or rectum may damage the nerves and blood vessels involved in erection. Prostate and bladder cancer surgery often require removing tissue and nerves surrounding a tumor, which increases the risk for impotence.)
- Aging.
- Lifestyle: alcohol and drugs, obesity, cigarette smoking (Incidence of impotence is approximately 85 percent higher in male smokers compared to non-smokers., Smoking is a key cause of erectile dysfunction. Smoking causes impotence because it promotes arterial narrowing.
- Overtraining
- Other disorders.
Surgical intervention for a number of different conditions may remove anatomical structures necessary to erection, damage nerves, or impair blood supply. Complete removal of the prostate gland or external beam radiotherapy of the gland are common causes of impotence; both are treatments for prostate cancer. Some studies have shown that male circumcision may result in an increased risk of impotence, while others have found no such effect, and another found the opposite.
Excessive alcohol use has long been recognised as one cause of impotence, leading to the euphemism "brewer's droop," or "whiskey dick;" Shakespeare made light of this phenomenon in ''Macbeth''.
A study in 2002 found that ED can also be associated with bicycling. The number of hours on a bike and/or the pressure on the penis from the saddle of an upright bicycle is directly related to erectile dysfunction.
Some evidence suggests that smaller penis size is associated with erectile dysfunction.
