Priapism is a medical condition that is characterized by a prolonged and usually painful erection of the penis. It is usually unrelated to sexual stimulation, and it can last for several hours. In some cases, it may even last for days. The most common underlying cause of priapism is an imbalance of the hormones epinephrine, norepinephrine, and serotonin. These hormones regulate blood flow to the penis. If they become imbalanced, they can cause an abnormal, sustained erection. Other causes of priapism can include sickle cell anemia and other blood disorders, use of certain medications such as antipsychotics and antidepressants, traumatic injury, alcohol or drug abuse, and use of certain recreational drugs. In some cases, an underlying disorder such as diabetes, multiple sclerosis, or Parkinson's disease may be responsible. In rare cases, priapism has been linked to certain types of cancer, such as prostate cancer and leukemia. Tumors located in or near the penis may also cause priapism. Severe low blood sugar, a condition called hypoglycemia, can also be a factor.