Laboratory evidence supports the existence of several placebo mechanisms and placebo effects in both healthy volunteers and patients with a variety of medical conditions. Furthermore, clinically relevant evidence shows that placebo effects can have meaningful therapeutic effects, because of their long magnitude and duration, in different patient populations. Although substantial progress has been made in understanding placebo effects, much laboratory and translational clinical trial research remains to be done, with the ultimate aim of harnessing placebo effects to improve patient care.
http://www.fisica.uh.cu/rationalis/etica-placebo/Lancet2010/Lancet2010.pdf