Enhancing Extinction Learning in Post Traumatic Stress Disorder (PTSD)

Inclusion Criteria: 1. Current primary PTSD diagnosis based on DSM-IV criteria, with a minimum duration of 12 weeks since the traumatic event. 2. Between the age of 18 and 65. Exclusion Criteria: 1. Current diagnosis of schizophrenia, delusional disorder, or organic mental disorder as defined by the DSM-IV. 2. Current diagnosis of bipolar disorder, depression with psychotic features, or depression severe enough to require immediate psychiatric treatment (i.e., serious suicide risk with intent and plan). 3. Current diagnosis of alcohol or substance dependence within the 3 previous months. 4. Unwilling or unable to discontinue current trauma-focused psychotherapy or psychotropic medication (at least 1 month medication free). 5. Ongoing intimate relationship with the perpetrator (in assault-related PTSD cases). 6. Unstable cardiovascular, autoimmune, endocrine, neurological, renal, hepatic, retinal, gastrointestinal, or hematological disorder or current seizure disorder. 7. Pregnant or nursing females; females of childbearing potential who are unwilling or unable to use an acceptable method of contraception for the duration of active treatment. 8. A history of hypersensitivity or allergy to MB. 9. Any condition possibly affecting drug absorption (e.g., gastrectomy). 10. Glucose-6-phosphate dehydrogenase deficiency. 11. Other severe acute or chronic medical or psychiatric condition or laboratory abnormality that, in the judgment of the Medical Directors, would make the participant inappropriate for entry.