The impact of psychological factors on placebo responses in a randomized controlled trial comparing sham device to dummy pill

To explore to what extent psychological factors such as expectation, depression, anxiety and belief in alternative medicine impact placebo response and differential responses to separate placebo interventions. We analysed data from a randomized controlled trial designed to compare the clinical response of two distinct placebo treatments (sham acupuncture device and placebo pill) in 119 participants with persistent distal upper arm pain due to repetitive stress injury. We used a multivariable linear regression model to identify potential correlates of self-reported upper extremity pain at the end of treatment in both placebo arms of the study combined. We also performed stratified analyses by placebo treatment. We did not find any of the psychological factors of interest to be associated with pain at the end of treatment in our combined analysis. We found higher baseline pain score and pain for longer than 1 year's duration to be significantly associated with higher pain scores at the end of treatment for the placebo treatments combined. In stratified analyses, for the sham acupuncture group, we found higher baseline depression score, higher baseline pain score and younger age to be independently correlated with higher pain score at the end of treatment. For the placebo pill group, only baseline pain was significantly correlated to pain score at the end of treatment. In this trial, neither expectancy nor psychological states were associated with response to placebo, with the exception of baseline depression score for the sham acupuncture arm.