Rationale MDMA-assisted psychotherapy is under investigation as a novel treatment for posttraumatic stress disorder (PTSD). The primary mechanism of action of MDMA involves the same reuptake transporters targeted by antidepressant medications commonly prescribed for PTSD. Objectives Data were pooled from four phase 2 trials of MDMA-assisted psychotherapy. To explore the effect of tapering antidepressant medications, participants who had been randomized to receive active doses of MDMA (75–125 mg) were divided into two groups (taper group ( n  = 16) or non-taper group ( n  = 34)). Methods Between-group comparisons were made for PTSD and depression symptom severity at the baseline and the primary endpoint, and for peak vital signs across two MDMA sessions. Results Demographics, baseline PTSD, and depression severity were similar between the taper and non-taper groups. At the primary endpoint, the non-taper group (mean = 45.7, SD = 27.17) had a significantly ( p  = 0.009) lower CAPS-IV total scores compared to the taper group (mean = 70.3, SD = 33.60). More participants in the non-taper group (63.6%) no longer met PTSD criteria at the primary endpoint than those in the taper group (25.0%). The non-taper group (mean = 12.7, SD = 10.17) had lower depression symptom severity scores ( p  = 0.010) compared to the taper group (mean = 22.6, SD = 16.69). There were significant differences between groups in peak systolic blood pressure ( p  = 0.043) and diastolic blood pressure ( p  = 0.032). Conclusions Recent exposure to antidepressant drugs that target reuptake transporters may reduce treatment response to MDMA-assisted psychotherapy.