To Test the Potential Efficacy of Repeated Intranasal Administration of Ketamine as a Treatment for PTSD

Inclusion Criteria: * Men or women, 18-65 years of age; * Participants must have a level of understanding sufficient to agree to all tests and examinations required by the protocol and must sign a written informed consent document; * Participants must fulfill DSM-5 criteria for current civilian or combat-related PTSD, based on clinical assessment by a study psychiatrist and on the CAPS -this is done to ensure at least moderate severity and to safeguard against high placebo response rates; * Women must be using a medically accepted reliable means of contraception (if using an oral contraceptive medication, they must also be using a barrier contraceptive) or not be of childbearing potential (i.e., surgically sterile, postmenopausal for at least one year); * Women of childbearing potential must have a negative pregnancy test at screening and prior to each intranasal administration; * Participants must be able to identify a family member, physician, or friend (i.e. someone who knows them well) who will participate in a Treatment Contract (and e.g. contact the study physician on their behalf in case manic symptoms or suicidal thoughts develop). Exclusion Criteria: * Women who plan to become pregnant, are pregnant or are breast-feeding (because the medical risk of using ketamine during pregnancy and breast-feeding is unknown); * Serious, unstable medical illnesses such as hepatic, renal, gastroenterologic, respiratory, cardiovascular, endocrinologic, neurologic, immunologic, or hematologic disease, including gastro-esophageal reflux disease, obstructive sleep apnea, history of difficulty with airway management during previous anesthetics, ischemic heart disease and uncontrolled hypertension, and history of severe head injury; * Clinically significant abnormal findings of laboratory parameters, physical examination, or ECG; * Patients with uncorrected hypothyroidism or hyperthyroidism; * Hormonal treatment (e.g., estrogen) started in the 3 months prior to the first intranasal administration day; * Use of evidence-based individual psychotherapy (such as prolonged exposure) during the study; * History of autism, mental retardation, pervasive developmental disorders, or Tourette's syndrome; * History of one or more seizures without a clear and resolved etiology; * History of (hypo)mania; * Past or current presence of psychotic symptoms, or diagnosis of a lifetime psychotic disorder including schizophrenia or schizoaffective disorder; * Drug or alcohol abuse or dependence within the preceding 3 months; a rather narrow time period was chosen, however, in order to allow participation by individuals with a history of substance abuse or dependence problems that could be secondary to their PTSD, and to more closely approximate patients seen in real-world settings; this is the same period of time that we used in our recently completed study of IV ketamine for PTSD. * Previous recreational use of ketamine or PCP; * Current diagnosis of bulimia nervosa or anorexia nervosa; * Diagnosis of schizotypal or antisocial personality disorder (since these are known to reduce the possibility of study completion); other Axis II diagnoses will be allowed; * Patients judged clinically to be at serious and imminent suicidal or homicidal risk. * A blood pressure of one reading over 160/90 or two separate readings over 140/90 at screen or baseline visits * Patients who report current treatment with a benzodiazepine, an opioid medication, or a mood stabilizer (such as valproic acid or lithium) within 2 weeks prior to randomization; patients taking stable doses of antidepressant medication for 3 months prior to randomization will be allowed. * For subjects who may participate in the MRI portion of the study, claustrophobia, any trauma or surgery which may have left magnetic material in the body, magnetic implants or pacemakers, and inability to lie still for 1 hour or more.