The goal of this clinical trial is to learn if Massed Prolonged Exposure Therapy (M-PE) can improve PTSD symptoms and reduce substance use in adults receiving intensive outpatient (IOP) treatment for substance use disorder (SUD). The main questions it aims to answer are: * Does M-PE reduce PTSD symptoms more effectively than trauma treatment as usual (TAU)? * Does M-PE reduce the number of days participants use substances? * How do patient-centered outcomes (such as depression, suicidal thoughts, and quality of life) differ between M-PE and trauma TAU? * Researchers will compare M-PE to treatment as usual to see if M-PE leads to better mental health and substance use outcomes and lower dropout rates. Participants will: * Attend multiple therapy sessions per week (M-PE) or receive usual care * Complete assessments at baseline, during treatment, end-of-treatment, 1-month, 3-month, and 6-month follow-up * Share feedback through surveys and interviews about their experience in the program