Dual diagnosis, also called co-occurring disorder, dual disorder, or comorbidity, means a person has a diagnosis of both a substance use disorder and a mental health disorder. Because one disorder can mask or heighten symptoms of the other, it is often difficult to determine which came first, and may make it more difficult to diagnose. It is estimated that more than 50% of those diagnosed with a serious mental health disorder are also dependent on drugs or alcohol. The Substance Abuse and Mental