This will be a 12-week randomized trial. Outpatients and patients from the Mood and Anxiety program at the Centre for Addiction and Mental Health (CAMH) with a current diagnosis of post-traumatic stressed disorder (PTSD) and cannabis-use disorder (CUD) will be randomized to receive individual motivational interviewing therapy and contingency management (n = 12) or individual motivational interviewing therapy alone (control group, n = 12) after enrolment.

Inclusion Criteria:

1. Subjects aged between 18 and 55 years (from both sexes [or genders]);
2. Diagnosed with CUD (moderate or severe) according to the Structured Clinical Interview for DSM-IV (SCID) (mode of cannabis consumption can be inhalation or ingestion);
3. Diagnosis of current (past 30 days) PTSD, using the CAPS-5 on screening;
4. On antidepressant medication(s) for at least 1 month (to ensure safety and homogeneity as PTSD treatment of our population);
5. Be able to provide written informed consent; and
6. Be able to communicate in English.

Exclusion Criteria:

1. diagnosed with a severe or unstable medical illness that precludes safe participation in the study as per the study physician;
2. diagnosed with schizophrenia, schizoaffective disorder, or bipolar disorder or current acute psychosis or mania based on DSM-5 criteria;
3. current suicidality risk as indicated by the Columbia Suicide Severity Rating Scale (C-SSRS) with concurrence after the study physician's evaluation if the response to C-SSRS questions 1 or 2 is "yes" 34; and
4. have pain interfering with normal function as reported by the 12-Item Short Form Survey (SF-12) (moderate or higher interference).