Objective: The nocebo effect represents a growing concern in clinical settings. Nocebo effects occur when the treatment context generates negative expectancies that trigger the experience or worsening of negative symptoms beyond any effects attributable to the treatment itself. Despite being identified in a range of outcomes and conditions, from pain to Parkinson's disease, there has not been an attempt to systematically quantify the nocebo effects across health outcomes. The purpose of the present review was thus to systematically review and meta-analyze the nocebo literature to quantify the size of the nocebo effect across outcomes and examine which factors moderate the size of the nocebo effect, including process of induction, treatment type, or health outcome.
Method: Systematic searches of PubMed, PychInfo, Medline, and Web of Science identified 130 ( = 8,219) independent eligible studies. To be included, studies had to include both a nocebo and control group/condition, which were compared to isolate the nocebo effect size.
Results: Overall, the magnitude of the nocebo effect was medium ( = 0.522) and highly heterogeneous. Two key moderators emerged: health outcome and process of induction. Here, the nocebo effect was medium for most somatic outcomes and affect, with no significant effect on worsening cognitive performance. Further, inducing nocebo effects through instruction in combination with conditioning produced larger nocebo effects.
Conclusions: The present review suggests nocebo effects can be reliably induced across somatic health outcomes, and interventions that target the effect of instructions will be of critical importance to reducing the occurrence of nocebo effects. (PsycInfo Database Record (c) 2023 APA, all rights reserved).
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