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Levine JD, Gordon NC, Bornstein JC, Fields HL. 
“Role of pain in placebo analgesia”. 
Proc Natl Acad Sci U S A. 1979 Jul;76(7):3528-31.
The hypothesis that perceived pain intensity can influence placebo analgesia was tested. One hundred and seven subjects rated their pain from from 0 to 10 on a visual analog scale after a standard wisdom tooth extraction. The expected course of such postoperative pain in the absence of therapy or placebo is a steady increase; this was confirmed by blind administration of the placebo. When placebos were given intravenously in view of the patients, some (placebo nonresponders) reported that their pain increased, whereas others (placebo responders) reported that their pain either decreased or remained the same over the next 60 min. A placebo response was more likely to occur if the pain rating 5 min prior to placebo administration (initial pain) was greater than 2.6. Furthermore, placebo responders with initial pain above this 2.6 level reported significantly greater mean analgesia than those with lower initial pain. Indeed, responders with initial pain less than 2.6 reported no change in pain during the 60 min after administration of a placebo. When their initial pain level was greater than 2.6, they reported a steady decline in pain over this period. However, above the 2.6 level there was no obvious relationship between the magnitude of the placebo analgesia and the initial pain.
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Apr 17, 2011 1:59
placebo anaglesia and endogenous opioids #

The abstract does not describe a key finding of this study. Subjects who experienced placebo analgesic effects reported a significant decrease in analgesia when pre-treated with the opioid antagonist naloxone. This is convincing evidence that placebo analgesic effects are at least partially mediated by endogenous opioids. See pp. 3529 ff. of the article for more information.
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