Psychological Predictors of Response to Open-Label Versus Double-Blind Placebo in a Randomized Controlled Trial in...

There is growing evidence that open-label placebo (OLP) may be an efficacious treatment of chronic and functional conditions. However, patient-level predictors of response to OLP have not been clearly identified. The aim of this study is to evaluate the psychological predictors of response to OLP and to compare this to double-blind placebo (DBP) and no-pill control (NPC).

This study is a secondary analysis of data collected in a 6-week randomized controlled trial evaluating placebo effects in irritable bowel syndrome (IBS). The primary outcome was change in IBS severity. Hierarchical linear regression identified predictors of placebo response in general and compared them between those randomized to OLP, DBP, and NPC. Predictor variables included personality traits, generalized anxiety, depression, visceral sensitivity (a measure of symptom-specific anxiety), and pain catastrophizing.

A total of 210 participants (mean age = 42.3 years, 73.3% female) were included. Regression models revealed that visceral sensitivity was a predictor of response to OLP and NPC but not DBP. Interestingly, the effects were opposite, with high visceral sensitivity predicting less improvement in NPC and more improvement in OLP. Pain catastrophizing was a negative predictor of response to OLP (i.e., high pain catastrophizing was associated with less improvement in OLP). Neither visceral sensitivity nor pain catastrophizing played a significant role for response to DBP.

IBS participants who score low on the Pain Catastrophizing Scale but high on the Visceral Sensitivity Index seem to benefit particularly from OLP. Our study suggests that different psychological mechanisms may be involved in DBP and OLP interventions.