BACKGROUND: Several authors have hypothesized that adverse drug events (ADEs) upon switching from reference biologics to biosimilar products are related to the nocebo effect. However, a thorough and current review of the existing literature has not been conducted. OBJECTIVE: To evaluate if patient and/or physician knowledge of a switch from a reference biologic product to a biosimilar product was associated with an increase in ADEs likely to be susceptible to the nocebo effect.METHODS: Studies reporting efficacy and safety outcomes of a switch from a reference product to a biosimilar product were reviewed. Biologics with FDA-approved biosimilars in the United States were considered for review, including adalimumab, bevacizumab, etanercept, and infliximab. Studies were identified by searching controlled vocabulary (e.g., MeSH terms) and keywords within MEDLINE (via PubMed) and Embase. Descriptive statistics were used to quantify subjective and objective complications in doubleblinded and single-blinded or open-label studies.RESULTS: Thirty-one trials including 3,271 patients were reviewed in the full analysis. Median discontinuation rates for any reason were 14.3% (range = 0.0-33.3) in open-label studies compared with 6.95% (range = 5.2-11.0) in double-blinded studies. Discontinuation rates for ADEs were 5.6% (range = 0.0-24.2) in open-label studies versus 3.1% (range = 2.0-5.2) in double-blinded studies, suggesting the nocebo effect does affect biosimilar adoption. Subgroup analyses of antidrug antibody (ADA) development and infusion reactions were similar between infliximab open-label and doubleblinded studies. Discontinuation rates for any reason, for ADEs, and for lack of efficacy were generally higher in infliximab open-label trials compared with double-blinded trials. Etanercept biosimilar discontinuation rates for any reason were similar between study designs; however, incidences of injection site reactions and discontinuation rates for ADEs were higher in double-blinded compared with open-label study designs.CONCLUSIONS: Current evidence is insufficient to confirm a biosimilar nocebo effect, although higher discontinuation rates in infliximab biosimilar open-label studies support this theory. Further studies are needed to evaluate the existence of a biosimilar nocebo effect.