I must admit I have a very hard time believing this exact claim as written. Do you have any sources?
'Partial antagonism of placebo analgesia by naloxone' (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/placebartic...): "placebos have actual biological effects on the brain and body and are more than response biases."
As it happens, one of the original placebo observations took place in a battlefield hospital that had run out of morphine. A physician began injecting saline, hoping the switch wouldn't cause too much psychological stress. To his surprise, many of his patients continued to report pain relief. Source:
http://www.brainfacts.org/sensing-thinking-behaving/mood/art...
"While treating wounded American soldiers during World War II, Henry Beecher ran out of pain-killing morphine. Desperate, he decided to continue telling the soldiers that he was giving them morphine, although he was actually infusing them with a saline solution. Amazingly, 40 percent of the soldiers reported that the saline treatment eased their pain."
Pharmaceutical studies of only using drug or not-drug have different characteristics.
I doubt that any serious researcher will make that claim. Such studies are, after all, based on self-reporting, notoriously unreliable and difficult to accurately quantify.
This is not to say that psychologists won't make claims like that. But psychological science is only peripherally related to the other kind of science -- the real kind.
Just wondering, how could you get rid of the placebo effect in the placebo experiment?
That's easy -- give one group a real placebo, give the other an imitation.
As for other sources, just type random related words into a search engine until something reliable pops out. You'll mostly find pop science articles including the word placebo though.