A penile erection is the hydraulic effect of blood entering and being retained in sponge-like bodies within the penis. The process is often initiated as a result of sexual arousal, when signals are transmitted from the brain to nerves in the penis. Erectile dysfunction is indicated when an erection is difficult to produce. There are various circulatory causes, including alteration of the voltage-gated potassium channel, as in arsenic poisoning from drinking water. The most important organic causes are cardiovascular disease and diabetes, neurological problems (for example, trauma from prostatectomy surgery), hormonal insufficiencies (hypogonadism) and drug side effects.

Psychological impotence is where erection or penetration fails due to thoughts or feelings (psychological reasons) rather than physical impossibility; this is somewhat less frequent but often can be helped. Notably in psychological impotence, there is a strong response to placebo treatment. Erectile dysfunction, tied closely as it is about ideas of physical well being, can have severe psychological consequences.

Besides treating the underlying causes such as potassium deficiency or arsenic contamination of drinking water, the first line treatment of erectile dysfunction consists of a trial of PDE5 inhibitor drugs (the first of which was sildenafil or Viagra). In some cases, treatment can involve prostaglandin tablets in the urethra, injections into the penis, a penile prosthesis, a penis pump or vascular reconstructive surgery.