Placebo response in trials with patients with anxiety, obsessive-compulsive and stress disorders across the lifespan: a three-level meta-analysis

Question Randomised controlled trials assessing treatments for anxiety, obsessive-compulsive and stress-related disorders often present high placebo response rates in placebo groups. Understanding the placebo response is essential in accurately estimating the benefits of pharmacological agents; nevertheless, no studies have evaluated the placebo response across these disorders using a lifespan approach. Study selection and analysis We searched MEDLINE, PsycINFO, Embase, Cochrane, websites of regulatory agencies and international registers from inception to 9 September 2022. The primary outcome was the aggregate measure of internalising symptoms of participants in the placebo arms of randomised controlled trials designed to assess the efficacy of selective serotonin reuptake inhibitors (SSRIs) and serotonin and norepinephrine reuptake inhibitors (SNRIs) in individuals diagnosed with anxiety, obsessive-compulsive or stress-related disorders. The secondary outcomes were placebo response and remission rates. Data were analysed through a three-level meta-analysis. Findings We analysed 366 outcome measures from 135 studies (n=12 583). We found a large overall placebo response (standardised mean difference (SMD)=-1.11, 95% CI -1.22 to -1.00). The average response and remission rates in placebo groups were 37% and 24%, respectively. Larger placebo response was associated with a diagnosis of generalised anxiety disorder and post-traumatic stress disorder, when compared with panic, social anxiety and obsessive-compulsive disorder (SMD range, 0.40-0.49), and with absence of a placebo lead-in period (SMD=0.44, 95% CI 0.10 to 0.78). No significant differences were found in placebo response across age groups. We found substantial heterogeneity and moderate risk of bias. Conclusions Placebo response is substantial in SSRI and SNRI trials for anxiety, obsessive-compulsive and stress-related disorders. Clinicians and researchers should accurately interpret the benefits of pharmacological agents in contrast to placebo response. PROSPERO registration number CRD42017069090.