Inability of the male to perform the sexual act. Impotence may be partial or complete, temporary or permanent. Psychological factors are the most common cause and these include anxiety, ignorance, fear, guilt, weakness of sexual desire or abnormality of such desire. Counselling or sex therapy, preferably with the partner often helps cure long-term impotence of psychological origin. Among organic causes are lesions (see LESION) of the external genitalia; disturbances of the ENDOCRINE GLANDS, such as diminished activity of the gonads, thyroid gland or pituitary gland; diseases of the central NERVOUS SYSTEM or spinal cord; and any severe disturbance of health, such as DIABETES MELLITUS.

An oral drug for treating erectile dysfunction is sildenafil citrate (Viagra®), the first in a class of drugs called phosphodiesterase type 5 inhibitors, also including tadalafil (Cialis®) and vardenafil (Levitra®). They work by improving blood flow to the penis, and can be taken an hour before intercourse (up to 12 hours before, in the case of tadalafil). Side-effects include headache, facial flushing and indigestion. There are certain restrictions on NHS prescribing. Intracavernosal injection or urethral application of alprostadil, a drug which increases local blood supply to the penis, has been used for some years under medical supervision, but success has been variable