This article presents the guidelines of the Polish Psychiatric Association regarding the co-occurrence of schizophrenia and substance use disorders (so-called dual diagnosis). This problem affects 40–60% of people with schizophrenia, and the most commonly used substances are nicotine, alcohol, cannabis, and stimulants. The co-occurrence of substance use disorder worsens the course of the illness, increases the number of relapses and hospitalizations, and raises the risk of suicide, violence, homelessness, and social exclusion, while also making it more difficult to adhere to treatment recommendations. The document emphasizes the need for integrated diagnosis, including a clinical interview, toxicological testing, information from family members, and standardized tools such as the SCID, PANSS, BPRS, AUDIT, DUDIT, DAST, and ASSIST. It is particularly important to differentiate schizophrenia from psychoses induced by psychoactive substances. Primary schizophrenia is indicated by the persistence of psychotic symptoms for at least one month after the end of intoxication or substance withdrawal. The guidelines recommend using the ICD-10/11 or DSM-5 criteria and conducting regular, integrated psychiatric and addiction assessments. The authors also point to the need to improve the organization of care and the implementation of comprehensive treatment for patients with dual diagnosis.