Abstract Background Using psychiatric drugs to treat drug dependence and its comorbidities is very common. The objective of this study was to analyze the interactions between prescribed drugs for patients treated at a specialized mental health care center for drug addicts, located in southern Brazil. Methods A cross-sectional study was conducted on secondary data collected from 2010 to 2018. We reviewed the medical records of patients aged 18 years or older who took psychotropic drugs and had any type of substance dependence. The analysis of psychotropic drug interactions was conducted in three databases: Medscape, Drug Interactions Checker, and Micromedex. Results We included 1,022 of the 2,322 patients attending the care center during the study period. Psychotropic drug interactions were found in 779 (76.4%) study participants, and they presented 2,292 (100%) interactions, out of which 136 (6.0%) had minor clinical risk, 537 (23.4%) had moderate risk, and 1,619 (70.6%) had major risk for the patient, totaling 172 incompatible combinations between two psychotropic drugs. Of the total number of interactions, 128 were pharmacokinetic and 44 were pharmacodynamic. Conclusions The high number of psychotropic drug interactions is a serious public health issue. Psychopharmacological treatment should be carefully addressed to be safe for the patient. Key messages The implementation of clinical pharmacy services in outpatient clinics, hospitals, and mental health care centers can be considered the most effective alternative to avoid adverse reactions. Pharmacists are ready to assist the entire multidisciplinary team involved in the treatment of patients with mental disorders or substance dependence, and provide pharmacological treatment.