Lessons learned from placebo groups in antidepressant trials

Abstract

This comprehensive review provides an overview about placebo and nocebo phenomena in antidepressant trials. Improvements in the placebo groups may partly be explained through methodological issues such as natural course of depression and regression to the mean, but also fundamentally reflect investigators' and participants' expectations. A meta-analysis by our group of 96 randomized placebo-controlled trials showed large placebo responses to antidepressant medication. Moderator analyses revealed substantially larger placebo responses in observer ratings compared with self-report. Effect sizes in observer ratings showed strong increase with publication year while this effect was not found for patients' self-ratings. This reflects the strong influence of investigators' expectations. The analysis of 'nocebo effects', e.g. adverse effects in placebo groups of antidepressant trials also confirms the impact of expectations: nocebo symptoms reflected the typical side-effect patterns expected in the drug group, with higher symptoms rates in the placebo groups of tricyclic antidepressant trials compared with placebo groups of trials testing selective serotonin reuptake inhibitors. While the placebo response seems to be similar for women and men, gender differences were found for nocebo rates. In the conclusion, we discuss potential implications for clinical trial designs and argue for interventions aimed at optimizing positive expectations of treatment benefit while minimizing the impact of adverse effects.