When viewing mental health illness and substance use disorders as comorbid conditions there are a number of different pathways to consider such as: mental illness and addiction as separate but contemporaneous conditions; mental illness as a risk factor for addiction; addiction as a risk factor for mental illness and temporary mental illness as a consequence of drug use.
People with mental illness experience more complications from drug use than the general population. A 'dual diagnosis' is when an individual has comorbid mental health and psychiatric diagnoses. 'Dual diagnosis' complicates the prognosis and treatment of each diagnoses though it has been shown that dual treatment of both conditions improves outcomes. Substance use can be an indicator of poor treatment adherence and an increased risk of failure of psychiatric treatment and one should also consider interactions between treatments for both conditions.
In summary it is important to distinguish between substance-induced psychiatric disorders and substance-related psychiatric disorders. Symptoms of psychiatric disorders such as depression, anxiety and psychosis are more the rule than the exception in patients who misuse substances. Treating both the psychiatric and substance disorders improves outcomes for patients. A lot of patients with a 'dual diagnosis' have experienced trauma and stigma so it is important to treat the patient holistically and with empathy and respect to achieve the very best outcomes.