Posttraumatic stress disorder (PTSD) is a condition that involves intense memories of a traumatic event and intense, persistent feelings of anxiety. There are several effective therapies for PTSD, but they are often time consuming. The investigators want to see if the investigators can shorten treatment time while keeping therapy effective by adding a medication called methylene blue, USP, taken orally as a pill, to the therapy. The specific aims are: 1) To see whether medication plus psychotherapy improves PTSD symptoms more than placebo plus psychotherapy or a waitlist; 2) To examine the long-term outcome of those receiving medication plus psychotherapy 1 and 3 months after treatment has ended; 3) To examine whether medication plus psychotherapy helps with depression, trauma-related cognitions, and functioning.

The psychotherapy the investigators are offering in this study is a type of cognitive behavioral therapy called imaginal exposure. In imaginal exposure, the investigators encourage the client to approach the memory of the trauma by recounting the trauma story to the therapist and discussing his or her reactions to the memory.

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.