This arm will receive Written Exposure Therapy in addition to residential SUD treatment.

This arm will receive five sessions of neutral writing in addition to residential SUD treatment.

Written Exposure Therapy (WET) is a five-session manualized treatment for PTSD. At the first session, psychoeducation about common reactions to trauma and PTSD are presented followed by a 30-minute written exposure. Following the exposure, a 10-minute check-in is completed, and individuals are instructed not to avoid any thoughts/feelings/ images related to the trauma between sessions. All subsequent sessions follow this structure: check-in on avoidance and feedback on writing, written exposure, and brief check-in post-writing about the writing process.

The PTSD Checklist for DSM-5 (PCL-5) is a 20-item self-report measure of symptoms of PTSD as defined by the DSM-5. Participants are instructed to keep in mind the identified worst event when responding to the items. Items are summed to create a total score which ranges from 0-80; a total score cut-off of 33 is recommended for identifying probable PTSD.

The Timeline Follow-Back (TLFB) method will be used to assess past 30-day substance use frequency and quantity. The TLFB will be administered by phone at 1-month and 3-month post-discharge. Days of use at Baseline will ask about the 30 days prior to entering treatment.

Biopac MP150 for Windows will be used to collect electrocardiogram (ECG) data at a sampling rate of 1000 Hz. ECG is measured using three disposable Ag/AgCl electrodes and data are processed using MindWare software. HR will be derived by spectral analysis of 1-min epochs with a Hamming windowing function and log transformed. HR will be measured continuously during the trauma-cue reactivity paradigm (primary outcome), and during WET treatment sessions (secondary outcome). Average HR in beats per minute will be calculated during the trauma-cue (primary) and for each 30-minute writing session (secondary).

Biopac MP150 for Windows will be used to collect electrodermal activity (EDA) data at a sampling rate of 1000 Hz. EDA is measured using two finger electrodes and data are processed using MindWare software. EDA will be measured continuously during the trauma-cue reactivity paradigm (primary outcome), and during WET treatment sessions (secondary outcome). SCR will be calculated as the maximum SC amplitude during the trauma-cue adjusted for the average SC amplitude during the final 30-seconds of the rest phase (primary outcome). SCR during each WET session (secondary outcome) will be calculated as the maximum EDA amplitude during the written exposure, adjusted for baseline amplitude.

The Short Inventory of Problems - Revised (SIP-R) is a 17-item self-report of adverse consequences associated with drug or alcohol use in four domains: physical, social, intrapersonal, interpersonal, and impulse control. Items are rated on a scale of 0 (Never) to 3 (Daily or Almost Daily) representing the frequency of each consequence. A total score is calculated by summing all items, with higher scores reflecting greater consequences.