To examine the clinical efficacy of the anti-glutamatergic medication riluzole in posttraumatic stress disorder (PTSD), and its effect on hippocampus biomarkers that our laboratory previously has identified using MRS.

Inclusion Criteria:

1. Male or female age 18-65.
2. Right handed
3. Able to provide written informed consent.
4. Meets DSM-5 criteria for PTSD (current month and past 3 months).
5. If already receiving psychiatric intervention(s), must be on a stable regimen.
6. Has normal physical examination, laboratory test results, and electrocardiogram results at pre-treatment visit.

Exclusion Criteria:

1. Unwillingness or inability to provide written informed consent.
2. Unable to swallow pills.
3. Unable to tolerate blood draws.
4. Currently enrolled in another intervention study.
5. Imminent risk for self-harm (assessed by a licensed clinician)
6. Active psychotic symptoms.
7. Current panic disorder.
8. Lifetime history of schizophrenia spectrum disorder, bipolar disorder, obsessive compulsive disorder, anorexia nervosa, seizure disorder, or neurologic disorder.
9. Urine toxicology positive for drug(s) of abuse.
10. Evidence of any clinically significant medical disease.
11. Women who test positive for ß-HCG, self-report as pregnant, or are nursing.
12. Substance or alcohol abuse within 3 months of Visit 1. Substance or alcohol dependence within 6 months of Visit 1.
13. Excessive caffeine use, defined as regular consumption of >700mg caffeine per day.
14. Treatment with an antipsychotic, valproate, other anticonvulsant, reversible MAOI within 4 weeks of Visit 1. Other excluded medications will be those with (1) known glutamatergic effects, (2) previous MRS evidence of effects on brain glutamate, (3) potential effects on riluzole levels or (4) risk of neutropenia - used within 4 weeks of the first study visit.
15. Previous exposure to riluzole or ketamine.
16. Treatment with any investigational medicine within 30 days of Visit 1.
17. Treatment with electroconvulsive therapy (ECT) within 3 months of Visit 1.
18. Uncorrected thyroid disease.
19. Any screening laboratory assay that is deemed to be a clinically significant abnormality by the investigator, with the exception of liver function tests (AST, ALT, alkaline phosphatase), which must be within 1.5 times the upper limit of normal.
20. Any contraindications to having an MRI scan.