see -> "open-hidden paradigm"
"Placebo Effects are Inherent in Clinical Practice – even when no placebo is given
Some of the clearest evidence supporting the involvement of placebo effects in clinical care comes in the form of the open-hidden paradigm (Figure 3). In this experimental paradigm a treatment is given in a routine manner (the open administration), where the psychosocial context surrounding treatment administration is present, and a hidden manner, where the treatment is given without the patient's knowledge. In the case of drug therapy, the open administration mimics normal clinical care, where the doctor injects a drug in full view of the patient with verbal and contextual interactions. In the hidden administration, the drug is infused by a computer pump in the absence of the clinician and the therapeutic context. Patients receiving hidden administration are aware that at some stage they will receive a drug but they do not experience the expectation component or other contextual factors surrounding the administration. Because the hidden administration removes the psychosocial context of treatment, this paradigm defines the placebo component as the difference between open and hidden administrations, although no placebo is given
The open-hidden paradigm has been used in several clinical settings.
In pain, hidden administrations of five commonly used painkillers (morphine, buprenorphine, tramadol, ketorolac, metamizol) have been demonstrated to be markedly less effective than open administrations. ..." -> http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2832199/