Placebo response in anxiety disorders.

OBJECTIVE: To review literature on placebo response in anxiety, to discuss sources and levels of placebo response in various anxiety disorders, and to suggest methods to prevent high placebo response rates in clinical research trials. DATA SOURCE: Data from scientific literature were identified using a MEDLINE search, and were extracted and summarized for this review. STUDY SELECTION: Representative findings were selected from clinical and epidemiologic studies, review articles, letters to the editor, book chapters, and proceedings. DATA EXTRACTION: Data from English-language reports of studies on humans were included. Only the most representative conclusions drawn from review articles were used. DATA SYNTHESIS: Anxiety disorders in general are thought to be extremely susceptible to a variety of influences, including patient characteristics and environmental variables. Reported placebo response levels in clinical studies of anxiolytics for generalized anxiety disorder and panic disorder vary widely, with a tendency to be rather high, although studies in social phobia and obsessive compulsive disorder appear to have consistently low placebo response rates. Comparisons of anxiety studies with studies of other indications, such as depression, show similar overall placebo response rates. To determine efficacy, drug response rates and placebo response rates must be clearly differentiated. CONCLUSIONS: Examination of the literature suggests that placebo response rates in studies of anxiolytics are influences by a number of factors, including both endogenous and exogenous variables. High placebo response rates may mask true drug response rates and may result from poor study design or lack of procedural standardization. The use of certain design methods may help to prevent high placebo response rates in anxiolytic clinical trials.