Differences in the placebo response in duloxetine and venlafaxine trials

Objective: Our analysis aimed at comparing the placebo effect sizes from randomized controlled trials (RCTs) of two widely prescribed antidepressants, namely duloxetine and venlafaxine, and at analysing a potential influence of the investigated drugs on the placebo response. Method: We conducted a comprehensive systematic review and meta-analysis of placebo-controlled, double-blind RCTs, which examined the efficacy of duloxetine and venlafaxine in the acute treatment of major depressive disorder. Results: We included 71 studies (29 duloxetine trials and 43 venlafaxine trials; one study provided data for the duloxetine and the venlafaxine data set). The placebo effect sizes, defined as pre-postscore change divided by baseline standard deviation, differed significantly between venlafaxine and duloxetine studies (-2.51 vs. -2.09; test for subgroup differences P = 0.028; high heterogeneity). The analysis of effect modifiers and the metaregression analyses confirmed the drug, next to baseline depression severity and publication status, as the most influential independent predictor. Conclusion: Our analyses show a significant difference in the placebo response between venlafaxine and duloxetine trials and suggest that the investigated drug has an influence on the placebo response that is not related to baseline severity, changes over the years or other variables we included.