Studies of pain in a particular system should be classified using that category e.g. gastrointestinal pain is classified as Oral and Gastrointestinal and fibromyalgia would be coded as Musculoskeletal. Studies focussed on pain pathways in the nervous system would be classified as Neurological. Other psychological studies of pain perception can be classified as Mental Health.

Basic studies on the mechanism of acute pain should be coded within the 1 Underpinning code group. However if the research is addressing pain from a specific condition or injury (including chronic pain) it would be appropriate to use other codes.