1. All treatment seeking adult (≥ 18 years)
2. Veterans with mTBI who meet PTSD diagnosis and have a CAPS-5 score of 25 or greater. Diagnosis will be determined using the provisional PTSD diagnosis convention recommended by the National Center of PTSD which requires at least moderate ratings (2 or more) on at least 1 B item (items #1-5), 1 C item (items #6-7), 2 D items (#8-14), and 2 E items (items #15-20) of the Clinician Administered PTSD Scale for DSM-5 (CAPS-5).

Pregnant or lactating women and those of child-bearing potential not using a reliable method of contraception; (2) Individuals meeting DSM-5 criteria for schizophrenia, schizoaffective disorder, or neurocognitive disorder; (3) Individuals currently taking any typical or atypical antipsychotic medication or individuals using daily benzodiazepines including nonbenzodiazepine z-drugs for sleep as they have effects similar to benzodiazepines for sleep. (Note: benzodiazepine use is contraindicated as they may impair learning required for progress with PE) (4) Individuals with known intolerance to quetiapine or a history of clinically unstable heart, lung, liver, renal or endocrinological condition, diabetes mellitus, and/or seizure disorder. (5) Individuals with substance use disorder severe enough to require medical detoxification.

(6) Individuals who report suicidal or homicidal ideations in the past month severe enough to warrant inpatient admission.

(7) Individuals with current or known history of cardiac arrhythmia or QTc interval ≥ 450 milliseconds. (Note) Patients requiring other anti-arousal medication for general medical conditions such as antihypertensive medications, beta blockers etc., for hypertension, anti-thyroid medications for hyperthyroidism, and/or anti-epileptic medications, divalproex, levetiracetam, or carbamazepine for seizure disorder may be included if stable for at least 1 month before the screening visit.