LAUSR

ABSTRACT Open-label placebos, or placebos without deception, have been found to induce analgesia, a challenging concept that need to be investigated in detail. In particular, what we need to know is the mechanism through which analgesia is induced when no deception is involved. Here we show for the first time that open-label placebo analgesia can be reversed by the opioid antagonist naloxone, as already shown for deceptive placebos. To do this, we used the tourniquet technique to induce experimental ischemic arm pain. The open-label placebo challenge started when pain scores reached 7 on a 0-10 rating scale. Whereas 59.4% of the subjects did not respond to the open-label placebo, 40.6% showed a substantial response. On the basis of the natural history control group, a placebo responder reported pain scores equal to or less than 7 after 9 min from the open-label placebo administration. In these responders, we found that a hidden injection of 10 mg naloxone could reverse placebo analgesia compared to a hidden injection of saline solution. At least two control groups showed that naloxone per se was not hyperalgesic, thus ruling out naloxone-induced hyperalgesia as a confounding variable. In light of the need to better understand open-label placebo effects, these findings represent the first experimental evidence that nondeceptive placebo analgesia may be mediated by the same mechanisms as deceptive placebo analgesia, namely, the endogenous opioid systems.