Adding cognitive behavior therapy (CBT) to a single selective serotonin reuptake inhibitor (SSRI) mildly improves symptoms of depression compared to the effect of the SSRI alone, a new study shows. But the full potential and effect of CBT may have been weakened by a reduction in subjects' use of SSRIs, according to Gregory Clarke, PhD, of the Kaiser Permanente Center for Health Research, where the study was performed.