The goal of this clinical trial is to compare the combination of Massed Prolonged Exposure (PE); a behavioral therapy for PTSD) and a stellate ganglion block (SGB; an injection of a local anesthetic into the front of the neck) with Massed Prolonged Exposure and a sham injection in a sample of military service members or retirees with PTSD. The main questions it aims to answer are: (1) Does the addition of an SGB improve treatment outcomes associated with Massed PE and (2) Do differences in psychophysiological arousal during the exposure portion of treatment help explain treatment outcomes for PTSD. Participants will receive ten 90-minute session of Massed PE. Between the first and second Massed PE sessions, half of the participants will receive a SGB, and half will receive a sham SGB.

Massed PE will be conducted by master-level or doctoral-level therapists. Participants will meet with their providers for individual, 90-minute sessions. They will then be asked to complete out-of-session treatment assignments throughout the rest of the day. Between the individual therapy session and out-of-session treatment assignments, participants will engage in approximately four to six hours of treatment per day, Monday through Friday, for two weeks. Each participant will also be offered three booster sessions at 1-, 3-, and 7-weeks posttreatment.

The stellate ganglion block injection or the sham SGB will be administered between the first and second massed PE session by qualified medical personnel as per standard operating procedure for the placement of a stellate ganglion block. A research nurse will be in attendance during the procedure and for an hour recovery period following the block administration.

Assessments will be administered at pretreatment, during treatment, at posttreatment, and at 1-, 3-, and 6-months following the completion of PE. The primary outcome assessment will be 1-month following the completion of PE. Following this assessment, participants randomized to the sham SGB arm of the study will be offered an SGB with ropivacaine.

Inclusion Criteria:

1. Active duty and retired military service members ages 18-65 years
2. PTSD diagnosis as assessed by Clinician-Administered Posttraumatic Stress Scale
3. Able to speak and read English (due to standardization of outcome measures)
4. Defense Enrollment Eligibility Reporting System (DEERS)-eligible to receive care at a military treatment facility (MTF) where the stellate ganglion block will be placed.

Exclusion Criteria:

1. Current suicidal ideation severe enough to warrant immediate attention (as determined by the Depressive Symptoms Index - Suicidality Subscale and the Self-Injurious Thoughts and Behaviors Interview short form) and corroborated by a clinical risk assessment by a credentialed provider.
2. Current manic episode or psychotic symptoms requiring immediate stabilization or hospitalization (as determined by clinical judgment)
3. Symptoms of moderate to severe substance use (to include alcohol) warranting immediate intervention based on clinical judgment.
4. Other psychiatric disorders severe enough to warrant designation as the primary disorder as determined by clinician judgment
5. Pregnancy or breastfeeding
6. Current anticoagulant use
7. History of bleeding disorder
8. Infection or mass at injection site
9. Myocardial infarction within 6 months of procedure
10. Pathologic bradycardia or irregularities of heart rate or rhythm
11. Symptomatic hypotension
12. Phrenic or laryngeal nerve palsy
13. History of glaucoma
14. Uncontrolled seizure disorder
15. History of allergy to local anesthetics
16. Current use of Class III antiarrhythmics

The clinical staff, research staff, and the participant will be blind to study condition until after the 1-month follow-up assessment. After which time, the blind will be broken, and participants in the control condition will be offered the SGB.