Endogenous opiates and the placebo effect - A meta-analytic review

Objective: A meta-analysis was performed to investigate the ability of placebo administration to reduce self-report of pain and to examine whether placebo-induced pain reduction might have physiological and psychological underpinnings. Method: Forty-five effect sizes and 1183 participants from 12 studies were meta-analyzed for the effects of placebo and the opioid antagonist, naloxone, on self-report of pain. Results: Analyses showed that placebo administration was associated with a decrease in self-report of pain, and a hidden or blind injection of naloxone reversed placebo-induced analgesia. Furthermore, there were significant between-group differences for type of pain (experimental vs. postoperative/clinical) for placebo studies. Conclusions: The results support the literature illustrating that the belief and expectation of analgesia induces discrete physiological changes, leading to relief from pain, and this response may be mediated by endogenous opioids. The implications of these findings are discussed in terms of the symbolic aspect of health care and mental health providers' words and context, and their potential impact on the course of illness and well-being. (c) 2005 Elsevier Inc. All rights reserved.