>Objectives: The aim of this study is to identify predictors of pill placebo response and to characterize the temporal course of pill placebo response in anxious youth.>Methods: Data from placebo-treated patients (N = 76) in the Child/Adolescent Anxiety Multimodal Study (CAMS), a multisite, randomized controlled trial that examined the efficacy of cognitive-behavioral therapy, sertraline, their combination, and placebo for the treatment of separation, generalized, and social anxiety disorders, were evaluated. Multiple linear regression models identified features associated with placebo response and models were confirmed with leave-one-out cross-validation. The likelihood of improvement in patients receiving pill placebo—over time—relative to improvement associated with active treatment was determined using probabilistic Bayesian analyses.>Results: Based on a categorical definition of response (Clinical Global Impressions-Improvement Scale score ≤2), nonresponders (n = 48), and pill placebo responders (n = 18) did not differ in age (p = 0.217), sex (p = 0.980), race (p = 0.743), or primary diagnosis (all ps > 0.659). In terms of change in anxiety symptoms, separation anxiety disorder and treatment expectation were associated with the degree of pill placebo response. Greater probability of placebo-related anxiety symptom improvement was observed early in the course of treatment (baseline to week 4, p < 0.0001). No significant change in the probability of placebo-related improvement was observed after week 4 (weeks 4–8, p = 0.07; weeks 8–12, p = 0.85), whereas the probability of improvement, in general, significantly increased week over week with active treatment.>Conclusions: Pill placebo-related improvement occurs early in the course of treatment and both clinical factors and expectation predict this improvement. Additionally, probabilistic approaches may refine our understanding and prediction of pill placebo response.
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