OBJECTIVE: There is a paucity of data on the effects of coprescribed benzodiazepines on treatment response variability and adherence to antidepressant pharmacotherapy for depression and anxiety in late life. The objective of this transdiagnostic analysis was to examine the effect of benzodiazepines on treatment outcomes in older patients with generalized anxiety disorder (GAD) or major depressive disorder (MDD). METHODS: Secondary analyses of data from 2 clinical trials of antidepressant pharmacotherapy for GAD (escitalopram vs placebo, 2006-2009) or MDD (open treatment with venlafaxine, 2009-2014) were conducted. Participants included 640 adults aged 60+ years with DSM-IV-defined GAD (n = 177) or MDD (n = 463). Benzodiazepine data were collected at baseline. Adherence and treatment response were assessed over 12 weeks. The analysis addressed whether coprescribed benzodiazepines are associated with treatment response, antidepressant medication adherence, dropout, final dose of antidepressant medication, and report of antidepressant-related adverse effects. RESULTS: Participants with GAD and coprescribed benzodiazepines were treated with a lower mean dosage of escitalopram and were less likely to complete the trial; there was no difference in adherence or treatment response. Participants with MDD and coprescribed benzodiazepines were less likely to tolerate a therapeutic dose of venlafaxine and reported more medication-related adverse effects; there was no difference in adherence, dropout, or treatment response. CONCLUSIONS: Coprescription of benzodiazepines was associated with increased dropout in older patients with GAD and more medication-related adverse effects in older patients with MDD. However, with the systematic clinical attention offered in a clinical trial, they do not impede treatment response. Clinicians should be aware that a coprescribed benzodiazepine may be a marker of a more challenging treatment course.