The relationships between substance use and substance-use disorders and comorbid mental disorders are complex, and are therefore difficult to examine in studies. Clinically, the two important questions to consider regarding the co-occurrence of two disorders in one patient are: firstly, whether there may be common underlying causes, and secondly, whether the comorbidity has an impact on clinical care.[1]
Reference:
[1] EMCDDA. Perspectives on drugs. Comorbidity of substance use and mental health disorders in Europe. 2016. Available at: https://www.emcdda.europa.eu/publications/insights/comorbidity-substance-use-mental-disorders-europe_en.