The high prevalence of cannabis and other substance use disorders are a major barrier to recovery in people with schizophrenia. Moreover, schizophrenia patients have significant deficits in cognitive function, which may be exacerbated by cannabis use. Complicating these problems is the lack of evidence-based treatments for co-morbid cannabis use disorders (CUDs) in schizophrenia; there are no established pharmacotherapies. Therefore, this study is investigating the effects of high-frequency (20Hz) repetitive transcranial magnetic stimulation (rTMS) on cannabis use disorder and cognitive function in patients with co-morbid schizophrenia/schizoaffective disorder. The proposed study would be the first randomized, double-blind, sham controlled trial of rTMS in patients with schizophrenia and co-morbid CUD. A total of N=40 schizophrenia smokers with co-morbid cannabis use disorder will be assigned to either active rTMS (N=20) or sham rTMS (N=20) as a treatment regimen of 5X/week treatment for four consecutive weeks. All participants will receive weekly behavioral therapy for 4 weeks. The investigators predict that active rTMS will be well-tolerated and superior to sham rTMS for the treatment of CUD in schizophrenia.

Inclusion Criteria:

1. Male (80%) or Female (20%);
2. Age 18-55;
3. Meet the Diagnostic and Statistical Manual of Mental Disorders version 5 (DSM-5) criteria for Schizophrenia (SZ) or schizoaffective disorder and cannabis use disorder with physiological dependence;
4. Full scale intelligence quotient (IQ) ≥ 80 determined through the Wechsler Test of Adult Reading (WTAR);
5. Non-smokers OR cigarette smokers as confirmed with Fragerstrom Test for Nicotine Dependence (FTND) score of 5 or higher, self reported smoking of at lest 5 cigarettes per day (measured by the Timeline Follow Back), and verified by a Smokerlyzer test, cut-off as 10 ppm.

Exclusion Criteria:

1. DSM-5 diagnoses of alcohol, substance or polyuse substance use disorder in the past 6 months (other than cannabis/caffeine or nicotine);
2. Currently active suicidal ideation or self-harm (suicidal or non-suicidal) as assessed by the Structured Clinical Interview for DSM-5 (SCID-5);
3. Head injury resulting in loss of consciousness (>5 minutes) and hospitalization;
4. Major neurological or medical illness including seizure disorder or syncope;
5. Metallic implants;
6. History of rTMS treatment;
7. Pregnancy.

The rTMS technicians, participants, research analysts and investigators involved in the study will be blind to the rTMS treatment assignment.