What trial participants need to be told about placebo effects to give informed consent: a survey to establish existing knowledge among patients with back pain.

<bold>Introduction: </bold>Patients require an accurate knowledge about placebos and their possible effects to ensure consent for placebo-controlled clinical trials is adequately informed. However, few previous studies have explored patients' baseline (ie, pretrial recruitment) levels of understanding and knowledge about placebos. The present online survey aimed to assess knowledge about placebos among patients with a history of back pain.<bold>Design: </bold>A 15-item questionnaire was constructed to measure knowledge about placebos. Additional questions assessed sociodemographic characteristics, duration and severity of back pain, and previous experience of receiving placebos.<bold>Setting: </bold>Participants recruited from community settings completed the study online.<bold>Results: </bold>210 participants completed the questionnaire. 86.7% had back pain in the past 6 months, 44.3% currently had back pain. 4.3% had received a placebo intervention as part of a clinical trial and 68.1% had previously read or heard information about placebos. Overall knowledge of placebos was high, with participants on average answering 12.07 of 15 questions about placebos correctly (SD=2.35). However, few participants correctly answered questions about the nocebo effect (31.9% correct) and the impact of the colour of a placebo pill (55.2% correct).<bold>Conclusions: </bold>The findings identified key gaps in knowledge about placebos. The lack of understanding of the nocebo effect in particular has implications for the informed consent of trial participants. Research ethics committees and investigators should prioritise amending informed consent procedures to incorporate the fact that participants in the placebo arm might experience adverse side effects. [ABSTRACT FROM AUTHOR]