There are multiple possible causes including:
- spinal cord injury,
- bladder stones,
- blood diseases (e.g. leukaemia in children),
- uncontrolled diabetes,
- some forms of widespread cancer,
- injury to the penis,
- excess dose of alprostadil (Caverject, Muse),
- illegal drugs (eg. cocaine, marijuana)
- prescription drugs (eg. prazosin, heparin).
It rarely occurs a result of excessive sexual stimulation.
Treatment involves warm packs, pseudoephedrine (Sudafed) tablets and syringes to draw excess blood from the penis. Any underlying cause needs to be excluded by appropriate tests. Permanent damage to the penis may occur due to the constant pressure on the tissue if it is not treated within four hours.
Priapus, the son of Aphrodite and Dionysus, had a perpetually erect huge penis and was the nymph of virility in Greek mythology.