Researchers have previously demonstrated that placebo analgesics have an effect on the opioid receptors in the brain, but insights into non-opioid placebo effects have remained elusive.

Fabrizio Benedetti and co-workers, University of Turin Medical School and National Institute of Neuroscience, Italy, have found that an antagonist, rimonabant (SR141716), blocks a specific CB1 cannabinoid receptor and so prevents non-opioid placebo analgesics from working. These apparently paradoxical findings in which a drug that blocks the pain-relief receptors in the brain stops the placebo effect from working, show that the endocannabinoid system may underpin some forms of placebo analgesia.

• Nonopioid placebo analgesia is mediated by CB1 cannabinoid receptors
  F. Benedetti, M. Amanzio, R. Rosato, C. Blanchard,
  Nature 2011.
  DOI: 10.1038/nm.2435