The penis normally becomes erect and hard with sexual stimulation, and subsides after ejaculation or cessation of stimulation. Rarely the penis may remain hard and erect when not stimulated, and this can cause considerable pain in the penis and the man cannot usually pass urine while the erection persists. This condition of prolonged and painful erection is called priapism.
If the penis remains erect and hard for several hours, there may be an inadequate blood supply to the tissues of the penis and these may become scarred and permanently damaged. Priapism should be dealt with if the erection has not subsided within two hours.
Treatment involves hot packs around the penis, taking the medication pseudoephedrine (Sudafed) found in many cold remedies, and as a last resort doctors can put a needle into the penis to drain out the excess blood.
Causes of priapism include an injury to the penis during sex or at an earlier time that has caused the formation of scar tissue, a calculus in the bladder, psychiatric conditions, damage to the spinal cord from an injury or tumour, drug abuse (e.g. alcohol, marijuana, cocaine), medications (e.g. prazosin, psychotropics, heparin, vasodilators) and an excessive dose of the drug alprostadil (Caverject) which is used to stimulate an erection.