Pain is an unpleasant feeling that is carried to the brain by the nervous system. Injury is a major cause, but pain may also arise from an illness. It may accompany a psychological condition, such as depression, or may even occur for no obvious reason.

Pain can be classified as acute or chronic (long-lasting) Acute pain often results from tissue damage, such as a skin burn or broken bone. Acute pain can also be associated with headaches or muscle cramps. This type of pain usually goes away as the injury heals or the cause of the pain is removed.

Chronic pain is pain that lingers after an injury heals, or is related to a disease, or has no known cause but will not go away. It is estimated that one in three people in the United States will experience chronic pain at some time in their lives.

Some...