The Placebo Analgesic Effect in Healthy Individuals and Patients: A Meta-Analysis.

Objective: The present meta-analysis investigates whether the magnitude of placebo analgesia is different in patients compared with healthy individuals and whether placebo analgesia is different in experimentally induced pain compared with clinical pain in patients.

Methods: A literature search in Web of Science (ISI) on the terms "placebo analgesia" and "placebo analgesic" was conducted. The search resulted in 71 studies, including 4239 participants. Fifty-five studies included healthy individuals and 16 studies included patients. Of the 16 studies with patients, five studies investigated clinical pain and 11 studies investigated experimentally induced pain.

Results: The average effect size was 1.24 for healthy individuals and 1.49 for patients. In the studies with patients, the average effect sizes of placebo treatment were 1.73 for experimentally induced pain and 1.05 for clinical pain. A χ test revealed that there were relatively more studies with patients compared with healthy volunteers in which there was a clinically significant reduction in pain (p = .040).

Conclusions: The findings suggest that patients benefited from placebo treatment to a greater degree than healthy individuals did and that studies on healthy individuals may underestimate the magnitude of the placebo analgesic effect in patients. Patients' clinical pain and experimentally induced pain respond to placebo to the same degree.