The purpose of this study is to determine the extent to which analgesic tolerance develops in
chronic pain patients who are either started on opioids or who receive an increase in
pre-study opioid dose level and then observed during long-term (20 weeks) stable opioid
therapy. In addition to clinical measures of relief of ongoing chronic pain, estimation of
tolerance development will be supplemented by periodic assessment of the anti-nociceptive
effects of opioids using the brief thermal sensitization model.