A Secondary Meta-Regression Analysis of the Relationship between Placebo and Nocebo Responses in ADHD Trials

Placebo and nocebo effects are psychological and physiological phenomena that significantly impact clinical trial outcomes, particularly in psychiatric disorders. While extensively studied individually, their interplay remains unexplored. This study investigates the relationship between placebo and nocebo responses in Attention Deficit Hyperactivity Disorder (ADHD) to improve the design and validity of Randomized Placebo-Controlled Clinical Trials (RPCCTs). This study is a secondary analysis of two previously published systematic reviews investigating placebo and nocebo responses in ADHD. A meta-regression analysis was conducted using data from 71 RPCCTs involving 6205 participants. Placebo response was measured as changes in ADHD symptom severity, while nocebo response was defined by the incidence of adverse events (AEs) in placebo groups. Multivariate analyses were used to explore the correlation between placebo and nocebo responses, adjusting for trial-level covariates. Placebo responses were positively associated with comorbidity as an inclusion criterion, psychotherapy, intention-to-treat analysis, and high risk of bias, and negatively associated with the number of study centers, use of a parallel design, and trials conducted in the U.S. Nocebo responses were positively associated with treatment naivety, treatment duration, proactive adverse event collection, and high risk of bias. The multivariate analysis showed no significant correlation between placebo and nocebo responses (coefficient = 0.0034, p-value = 0.8881) in ADHD trials. These findings challenge assumptions regarding the interdependence of placebo and nocebo responses and highlight the need for independent consideration of these phenomena in clinical trial design. Understanding these mechanisms can contribute to refining trial methodologies and optimizing therapeutic outcomes for ADHD. Further research is needed to explore whether this lack of correlation extends to other psychiatric disorders.