Cognitive behavioural therapy (CBT) is an evidence-based treatment for anxiety disorders. Patients engaging in CBT often have ongoing pharmacological treatment, generally antidepressants. Benzodiazepines are also commonly used, although they are recommended as short-term adjunctive treatment due to side effects. This mixed methods study, embedded in a randomized controlled trial of group transdiagnostic CBT (tCBT), explored the influence of benzodiazepine use prior to tCBT and the participants' experience of their use before, during, and after therapy. The experimental arm received tCBT and treatment-as-usual (TAU; n = 117) for 12 weeks while the control arm received TAU (n = 114). Anxiety symptoms were assessed with the Beck Anxiety Inventory (BAI). Multiple linear regression analysis examined BAI pre-post differences with benzodiazepine use in the past 12 months, tCBT and their interaction. Semistructured interviews were conducted with 13 participants reporting the use of benzodiazepines at baseline and analyzed using an interpretive descriptive method. Quantitative results suggested that tCBT had a significant effect on anxiety symptoms change, but this effect was nondifferential according to benzodiazepine use in the past 12 months. Qualitative results provided nuances about the perceived helpfulness of benzodiazepines. Participants reported that benzodiazepines facilitated the experience of sharing in group therapy and conducting exposure exercises. Despite some benefits, participants perceived that benzodiazepines may have interfered with the effectiveness of exposure exercises and the acquisition of concepts due to a sedative effect. This study highlight the importance of sensitizing therapists and patients to the potential effects of benzodiazepines on the experience of tCBT. Public Significance StatementParticipants expressed a need for guidance regarding their use of benzodiazepines, especially when they perceived that this medication could potentially interfere with some psychotherapeutic interventions. Therapists should be aware of the potential effects of the utilization of benzodiazepines on psychotherapy processes and address this issue in their intervention plan, considering that many people choose to engage in cognitive behavioural therapy (CBT) while undergoing pharmacological treatment.