Objective: Cognitive-behavioral therapy (CBT) is frequently used for various adult anxiety disorders, but there has been no systematic review of the efficacy of CBT in randomized placebo-controlled trials. The present study meta-analytically reviewed the efficacy of CBT versus placebo for adult anxiety disorders.

Data sources: We conducted a computerized search for treatment outcome studies of anxiety disorders from the first available date to March 1, 2007. We searched MEDLINE, PsycINFO, PubMed, Scopus, the Institute of Scientific Information, and Dissertation Abstracts International for the following terms: random*, cognitive behavior*therap*, cognitive therap*, behavior*therap*, GAD, generalized anxiety disorder, OCD, obsessive compulsive disorder, social phobia, social anxiety disorder, specific phobia, simple phobia, PTSD, post-traumatic stress disorder, and acute stress disorder. Furthermore, we examined reference lists from identified articles and asked international experts to identify eligible studies.

Study selection: We included studies that randomly assigned adult patients between ages 18 and 65 years meeting DSM-III-R or DSM-IV criteria for an anxiety disorder to either CBT or placebo. Of 1165 studies that were initially identified, 27 met all inclusion criteria.

Data extraction: The 2 authors independently identified the eligible studies and selected for each study the continuous measures of anxiety severity. Dichotomous measures reflecting treatment response and continuous measures of depression severity were also collected. Data were extracted separately for completer (25 studies for continuous measures and 21 studies for response rates) and intent-to-treat (ITT) analyses (6 studies for continuous measures and 8 studies for response rates).

Data synthesis: There were no significant differences in attrition rates between CBT and placebo. Random-effects models of completer samples yielded a pooled effect size (Hedges' g) of 0.73 (95% CI = 0.88 to 1.65) for continuous anxiety severity measures and 0.45 (95% CI = 0.25 to 0.65) for depressive symptom severity measures. The pooled odds ratio for completer treatment response rates was 4.06 (95% CI = 2.78 to 5.92). The strongest effect sizes were observed in obsessive-compulsive disorder and acute stress disorder, and the weakest effect size was found in panic disorder. The advantage of CBT over placebo did not depend on placebo modality, number of sessions, or study year.

Conclusions: Our review of randomized placebo-controlled trials indicates that CBT is efficacious for adult anxiety disorders. There is, however, considerable room for improvement. Also, more studies need to include ITT analyses in the future.