The purpose of this study is to evaluate the safety and efficacy of vaporized cannabis in participants with chronic, treatment-resistant posttraumatic stress disorder.

Inclusion Criteria:

1. Meet DSM-5 criteria for chronic PTSD of at least six months duration.
2. Have PTSD of moderate severity as measured by a score of >= 40 on the PCL-5 at the time of baseline assessment.
3. Have treatment resistant PTSD defined as meeting DSM-5 diagnostic criteria for PTSD after failing on, or being unable to tolerate, Health Canada-approved medication or empirically supported psychotherapy for PTSD of adequate dose and duration, as determined on a case-by-case basis by the site investigators.
4. Are at least 18 years old.
5. Are willing to commit to medication dosing and delivery method, to completing evaluation instruments, and to attending all study visits.
6. Agree to use only cannabis provided by study staff until the end of Stage 2 and agree to required cessation periods for the duration of the study.
7. Report no current hazardous cannabis use, as defined by a score of < 11 on the CUDIT-R at time of screening.
8. Abstain from cannabis during the 8-week baseline assessment period as biochemically verified via urine cannabinoid concentrations.
9. Agree to keep all study cannabis stored in a secure location and not to share/distribute cannabis to any other individual.
10. Be stable on medications and/or psychotherapy for PTSD for at least one month prior to study entry.
11. Agree to report any changes in medication or psychotherapy treatment regimen during the study to study staff.
12. If female and of childbearing potential, agree to use an effective form of birth control during study participation.
13. Participants must be proficient in reading English, and must be able to effectively communicate with the investigators and other site personnel.
14. Agree not to participate in any other interventional clinical trials during study participation.
15. Agree not to donate blood from the start of study treatment to 24 hours after the last dose.
16. Agree to allow the collection of his/her gender, race, and occupation to ensure that the study recruits the targeted population.

Exclusion Criteria:

1. Are pregnant or nursing, or of child bearing potential and not practicing an effective means of birth control.
2. Have a history of primary psychotic disorder, bipolar affective disorder, bipolar disorder with psychotic features, depressive disorder with psychotic features, borderline personality disorder, antisocial personality disorder, or positive family history (first degree relative) of psychotic disorder or bipolar affective disorder.
3. Have any allergies to cannabis or contraindication for using cannabis.
4. Are currently taking drugs known to be substrates for CYP 3A4 or CYP 2C19, such as amitriptyline, fentanyl, sufentanil, and alfentanil.
5. Have a diagnosis of obstructive sleep apnea or a score of >3 on the STOP-Bang questionnaire (except in cases where the participant has documented evidence of not having obstructive sleep apnea OR if the participant is compliant on CPAP treatment). Documented evidence consists of a negative result for obstructive sleep apnea on the completion of a formal assessment for apnea.
6. Would present a serious suicide risk as assessed by the investigators, or who are likely to require psychiatric hospitalization during the course of the study.
7. Are not able to give adequate informed consent.
8. Are not able to attend face-to-face visits or plan to move out of the area during the active treatment period.
9. Have a positive urine drug screen for opiates (unless prescribed or contained in an over-the-counter Health Canada approved medication), methamphetamine, cocaine and amphetamines or meet the DSM-5 criteria for substance use disorder (other than caffeine or nicotine) during Stage 1 and 2 of the study.
10. Have signs of ischemia (defined as ST elevation or depression) or significant arrhythmia (defined as atrial fibrillation or flutter, ventricular fibrillation or flutter) on the screening electrocardiogram.
11. Have abnormal hepatic or renal function (abnormal liver function tests or elevated creatinine results on the screening laboratory reports).
12. During the 8-week screening period, are diagnosed with dissociative identity disorder or an eating disorder with active purging, evidence of significant, uncontrolled hematological, endocrine, cerebrovascular, cardiovascular, coronary, pulmonary, gastrointestinal, or neurological disease;
13. During the 8-week screening period, meet criteria for cannabis use disorder (4 or more of 11 DSM-5 criteria) and continued cannabis use confirmed by urine testing.