This trial is testing if writing about traumatic experiences (WET) helps Veterans with both substance use issues and PTSD more than other treatments. Veterans will either write about their trauma or a neutral topic over several sessions. The study aims to see if WET can improve PTSD and substance use outcomes. Written Exposure Therapy (WET) is a treatment for PTSD developed through systematic evaluations of expressive writing, requiring limited patient and therapist time with no between-session assignments.