Impact of placebo assignment in clinical trials of tic disorders.

Background: Understanding the impact of placebo treatment is pivotal to the correct interpretation of clinical trials. The aim of present study was to examine the placebo effect in tic disorders.

Methods: Raw data were obtained for 6 placebo-controlled parallel and cross-over trials that involved medical interventions for tic disorders. Tic severity was measured using the Yale Global Tic Severity Scale. Placebo effect was defined as an improvement of at least 30% over baseline scores in the total tic score and was considered clinically relevant when at least 10% of patients in the placebo-arm met that benchmark.

Results: In total, 91 placebo-treated patients (80% males; mean age, 16.5 years; standard deviation, 10.5 years) were included. Although there was a trend toward improvement in the total tic scores after placebo administration (P=0.057), the magnitude of the placebo effect was small (Cohen's d=0.16) but relevant (19% of the sample). Females were more likely than males to have a placebo effect.

Conclusions: The magnitude of the placebo effect in tic disorders appeared to be small. Further longitudinal studies using objective assessments for tic disorders are warranted to confirm the current results. © 2013 Movement Disorder Society.