ArticleIncreased placebo analgesia over time in irritable bowel syndrome (IBS) patients is associated with desire and expectation but not endogenous opioid mechanismsVase, Lenea; Robinson, Michael E.b,*; Verne, Nicholas G.c; Price, Donald D.dAuthor Information aDepartment of Psychology, University of Aarhus, Aarhus, Denmark bDepartment of Clinical and Health Psychology, University of Florida, HPNP, 101 S. Newell Dr. 3151, P.O. Box 100165, Gainesville, FL 32610-0165, USA cDepartment of Gastroenterology, Veterans Administration Hospital, Gainesville, FL, USA dDepartments of Neuroscience and of Oral and Maxillofacial Surgery, University of Florida, Gainesville, FL, USA *Corresponding author. Tel.: +1 352 395 0490; fax: +1 352 395 0468. E-mail: [email protected] Submitted July 19, 2004; revised March 2, 2005; accepted March 14, 2005. Pain: June 2005 - Volume 115 - Issue 3 - p 338-347 doi: 10.1016/j.pain.2005.03.014 Buy Metrics Abstract A study was conducted to determine whether changes in expected pain levels, desire for pain relief, or anxiety contribute to an increase in placebo analgesia over time as well as to determine whether placebo analgesic effects of IBS patients are related to endogenous opioid mechanisms. Twenty-six women with IBS were exposed to rectal stimulation (35 or 55 mmHg for 30 s) and tested under natural history (NH), rectal placebo (RP) and rectal lidocaine (RL) conditions. During all conditions, 16 patients were given saline intravenously (to test for a placebo effect) and 10 patients were given naloxone intravenously (to test naloxone antagonism of the placebo effect) on a double blind basis. Patients rated expected pain level, desire for pain relief and anxiety at 2 and 22 min after the onset of NH, RP, and RL conditions and they rated actual pain intensity at 5-min intervals for 40 min. There was a large and significant placebo effect (P<0.001) that increased over time. Ratings of expected pain levels, desire for pain relief and anxiety decreased over time and contributed to more variance in placebo and lidocaine responses during the last half of the session. These changes suggest that a reduction in negative emotions may be central to placebo effects. There was no significant difference between psychological mediators (desire, expectation, anxiety) or the placebo effect in the saline and naloxone groups, indicating that neither the psychological mediators nor the placebo analgesic effect were associated with endogenous opioids in this clinically related paradigm. © 2005 Lippincott Williams & Wilkins, Inc.