Some related studies: 'Partial antagonism of placebo analgesia by naloxone' (http://www.ncbi.nlm.nih.gov/pubmed/6308540 http://www.ncbi.nlm.nih.gov/pubmed/6308

Some related studies: 'Partial antagonism of placebo analgesia by naloxone' (http://www.ncbi.nlm.nih.gov/pubmed/6308540 http://www.ncbi.nlm.nih.gov/pubmed/6308540) -- no actual morphine was ever adminstered, only a regimen of saline injections, whose placebo effect is apparently achieved by the release of endogenous opioids. 'A comprehensive review of the placebo effect'(http://pharmacology.ucsd.edu/graduate/courseinfo/placebarticle.pdf http://pharmacology.ucsd.edu/graduate/courseinfo/placebartic...): "placebos have actual biological effects on the brain and body and are more than response biases."