Key Concepts: Psychoactive (mind-acting) drugs affect areas in the brain which are also affected by mental illness. Therefore it is not surprising that substance misuse and abuse can co-occur within the same individual. Comorbidity defines this co-occurrence of substance addiction and other mental illnesses. It is important to have a basic awareness of defines psychoactive drugs, how they produce their physiological effects, and within the context of a system of care, how to address the needs of someone experiencing the consequences of inappropriate substance use, especially coupled with mental illness.