Placebo effects (plural) are mostly reporting bias, regression to the mean, investment justification, researcher bias, and other sources of self-deception. They are transient, and significant only for subjective symptoms where reporting bias can play a major role. Studies have shown, in fact, that there is no significant "healing" that occurs due to placebo effects — no objective biological improvement.

NeuroLogica Blog, Can Thinking Change Reality Part II (March 11, 2014)