Erectile dysfunction (ED) or impotence is sexual dysfunction characterized by the inability to develop or maintain an erection of the penis during sexual activity. A penile erection is the hydraulic effect of blood entering and being retained in sponge-like bodies within the penis. The process is most often initiated as a result of sexual arousal, when signals are transmitted from the brain to nerves in the penis. 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 can often be helped. Notably in psychological impotence, there is a strong response to placebo treatment. Erectile dysfunction can have severe psychological consequences as it can be tied to relationship difficulties and masculine self-image generally. 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. The Latin term impotentia coeundi describes simple inability to insert the penis into the vagina; it is now mostly replaced by more precise terms, such as erectile dysfunction (ED). The study of erectile dysfunction within medicine is covered by andrology, a sub-field within urology. Research indicates that erectile dysfunction is common, and it is suggested that approximately 40% of males suffer from erectile dysfunction or impotence, at least occasionally.