Comparison of psychological placebo and waiting list control conditions in the assessment of cognitive behavioral therapy for the treatment of generalized anxiety disorder: a meta-analysis

BACKGROUND: There is ongoing debate about the efficacy of placebos in the treatment of mental disorders. In randomized control trials (RCTs) about the treatment of generalized anxiety disorder, the administration of a psychological placebo or placement on a waiting list are the two most common control conditions. But there has never been a systematic comparison of the clinical effect of these different strategies. AIM: Compare the change in symptom severity among individuals treated with cognitive behavioral therapy, provided a psychological placebo, or placed on a waiting list using data from RCTs on generalized anxiety disorder. METHODS: The following databases were searched for RCTs on generalized anxiety disorder: PubMed, PsycInfo, EMBASE, The Cochrane Library, CNKI, Chongqing VIP, Wanfang, Chinese Biological Medical Literature Database, and Taiwan Electronic Periodical Services. Studies were selected based on pre-defined inclusion and exclusion criteria and the quality of each included study – based on the risk of bias and the level of evidence – was formally assessed. Meta-analysis was conducted using RevMan5.3 and network meta-analyses comparing the three groups were conducted using R. RESULTS: Twelve studies with a combined sample size of 531 were included in the analysis. Compared to either control method (placebo or waiting list), cognitive behavioral therapy was more effective for generalized anxiety disorder. Provision of a psychological placebo was associated with a significantly greater reduction of symptoms than placement on a waiting list. Eight of the studies were classified as ‘high risk of bias’, and the overall level of evidence was classified as ‘moderate’, indicating that further research could change the overall results of the meta-analysis. CONCLUSIONS: RCTs about the treatment of generalized anxiety disorders are generally of moderate quality; they indicate the superiority of CBT but the results cannot, as yet, be considered robust. There is evidence of a non-negligible treatment effect of psychological placebos used as control conditions in research studies. This effect should be considered when designing and interpreting the results of randomized controlled trials about the effectiveness of psychotherapeutic interventions.