The cause of chronic pain is not clear. When you have an injury or illness, certain nerves send pain signals to your brain. With chronic pain, these pain signals continue for weeks, months, or even years after you recover. Chronic pain can develop after a major injury or illness, such as a back injury or shingles, or it can develop without a known cause. It is also possible that certain brain chemicals that usually suppress pain may not work properly. The pain may be:

• Neurogenic pain, or neuropathic pain, which occurs when the peripheral nerves or central nervous system are somehow damaged. The nerves themselves cause the pain, and this kind of pain may not respond well to treatment.
• Psychogenic pain, which is pain that may be caused by an emotional or mental health issue. This pain is not caused by a disease, injury, or damage to the nervous system. Psychogenic pain is not common, but stress, depression, and other mental health factors can make the pain worse.
• Unidentifiable pain. It may be impossible to find or identify the cause of your pain. Tests may not reveal any injury, illness, or tissue change that could have triggered the pain.