A co-occurring disorder is a condition that exists secondarily to another major health issue. If we use this co-occurring disorders definition to look at mental illness and substance abuse, we can see that turning to drugs or alcohol, which then results in addiction, often happens simultaneously when a patient also has mental health concerns. Since one issue influences and worsens the other, we would call these co-occurring disorders. Co-occurring disorders must be treated together through a specialized program to ensure full recovery.