There is an abundant of evidence to support that our beliefs and expectations influence both benefits and adverse effects of any given drug.
This study also used functional MRI to record brain activity on healthy volunteers to confirm the effects of expectations on the analgesic efficacy of remifentanil (a short-acting synthetic opioid) during experimentally induced pain.
In summary: Positive treatment expectancy doubled the analgesic benefit of the opioid and activated the endogenous pain modulatory system as seen on brain imaging. In contrast, negative treatment expectancy abolished its analgesic effects and activated the hippocampus.
Reference: Bingel U et al The effect of treatment expectation on drug efficacy: imaging the analgesic benefit of the opioid remifentanil. Sci Transl Med. 2011 Feb 16;3(70):70ra14.