Antidepressants are adjuvant analgesics, or medications that treat pain even though doing so is not their primary purpose. Antidepressants are not generally used to treat chronic muscular aches and pains, but they are often effective for conditions like neuropathic pain and pain syndromes, such as fibromyalgia and central pain syndrome.
In part, antidepressants for chronic pain may work at the level of the nerve endings in the spinal cord, changing the way the brain perceives pain. Some also help to regulate sleep and control anxiety and depression, which may be contributing factors to the level of chronic pain. Often, however, antidepressants for chronic pain are prescribed at lower doses than would be therapeutic for clinical depression.
American Chronic Pain Association. APCA Medications and Chronic Pain: Supplement 2007. Accessed 6/21/09. http://www.theacpa.org/documents/ACPA%20Meds%202007%20Final.pdf