It isn't always clear what causes chronic pain. There are several reasons:

• injury: Even after the injury has healed, the nerves can continue to send pain signals to the brain. The medical community isn't sure why this happens.

• disease: Some conditions cause chronic pain. Fibromyalgia and osteoarthritis are some examples.

• nerve problems: You can injure part of your nervous system—the nerves themselves. This type of chronic pain, called neuropathic pain, doesn't respond well to treatment.

• unknown: Pain can develop, even with no obvious injury, disease, or nerve issue.