If a man's erection lasts for more than four hours even though he is no longer sexually excited, this is a medical problem called priapism (). The man must go to a doctor or hospital quickly if it happens. The condition is named after the Greek god Priapus, whose penis was believed to always be erect (see below). Priapism is painful, and is a medical emergency because the blood stuck in the penis can become thick and form clots. This can damage the blood vessels of the penis and lead to erectile dysfunction (ED). In serious cases the tissues of the penis can die, and the penis may have to be amputated (cut off) so that the tissue death does not spread to the rest of the body.