Schizophrenia is a severe psychiatric disorder that affects 1% of population worldwide. Regarding specific substance tobacco, alcohols, cannabis & cocaine use disorders occur commonly in patients with schizophrenia. Importantly, this population is associated with violence, suicide and increased rate of hospitalization while the exact mechanism contributing to substance use in Schizophrenia are not known, a number of theories have been put forward to explain the basis of co-occurrence of these disorders.