Abstract: Lack of evidence to guide medication treatments for mild traumatic brain injury (mTBI) in veterans too often results in polypharmacy practices attempting to provide symptomatic relief from multiple postconcussive syndrome symptoms. Therefore, the field needs to find an effective medication that reduces the burden of postconcussive symptoms without complicating the treatment burden of veterans. This clinical trial seeks to determine whether switching veterans to quetiapine monotherapy (intervention) is superior to continuing to receive treatment as usual (TAU, control) polypharmacy for veterans with symptoms of postconcussive syndrome and posttraumatic stress disorder who are receiving rehabilitation treatment for mTBI. This study will test the conceptual mediation model hypothesis that quetiapine monotherapy may enhance recovery from mTBI by (1) increasing engagement in rehabilitation services, and/or (2) reducing the adverse effects of TAU polypharmacy. This study will enroll 146 patients from two Veterans Administration Medical Centers into a 12- week phase III, randomized, pragmatic clinical trial comparing outcomes from treatment with quetiapine monotherapy and TAU. Quetiapine will be cross tapered up to a maximum dose of 200 mg (as tolerated) as other medications are discontinued. The primary outcome measures are postconcussive syndrome symptoms (Neurobehavioral Symptom Inventory), functional disability (World Health Organization Disability Assessment), and quality of life (World Health Organization Quality of Life Assessment). Overall, this study aims to determine whether quetiapine monotherapy is superior to TAU polypharmacy and improves the quality of life for veterans with comorbid postconcussive syndrome and posttraumatic stress disorder symptoms who are receiving rehabilitation treatment for mTBI.