Dose Optimization of MDMA-Assisted Therapy for PTSD

This open label, within-subjects dose optimization study will determine the optimal number of treatment cycles of MDMA-AT (one cycle consists of one Experimental Session followed by three Integrative Sessions) for significant improvement in a sample of U.S. veterans with PTSD. All participants will receive three non-drug preparatory sessions and at least one cycle of MDMA-assisted therapy with a team of two therapists. Each cycle will consist of one approximately 8-hour drug session followed by three 90-minute non-drug integrative sessions. After each cycle, participants will determine, in consultation with their therapy team, whether to continue with an additional cycle or to discontinue treatment. The decision will reflect the participant's perception of the potential for continued improvement (e.g., as based on current symptom burden, treatment response to date) weighed against the burden of an additional cycle (e.g., time required, any adverse events or emotional burden). Participants may complete up to a maximum of five cycles. The dose optimizing design allows for the identification of the optimal number of MDMA-AT cycles to achieve the best response and to assess tolerability of multiple cycles of MDMA-AT. Participants will also provide blood samples to be banked for future analysis of biological markers associated with treatment response. For each participant, the study will consist of: * Screening Period: initial screen, informed consent, eligibility assessment * Enrollment Period with Enrollment Confirmation: Enrollment and medication tapering period (if needed) followed by psychological assessment and Enrollment Confirmation, collection of biomarker blood samples. * Treatment Period: 3 Preparatory sessions; one to five Experimental Sessions, each followed by 3 non-drug Integrative Sessions and a follow-up visit with study therapists * Post-treatment Period and Study Termination: Psychological Evaluation, biomarker blood samples, study termination visit