Dual diagnosis refers to when someone has both a substance use disorder as well as a psychological disorder. Also known as co-occurring disorders, such individuals need to be treated for both the addiction and mental illness simultaneously in order for them to get better. Sometimes psychiatric disorders among drug or alcohol abusers disappear with prolonged abstinence. Other times the addiction is a secondary problem and symptomatic of deeper issues; addictive behavior can be a way to self-medicate untreated mental health issues.

Diagnosing a primary psychiatric illness in substance abusers is challenging as drug abuse itself often induces psychiatric symptoms, thus making it necessary to differentiate between substance-induced and pre-existing mental illness.  Substance-induced psychiatric symptoms can occur both in the intoxicated state and also during the withdrawal state. In some cases these substance-induced psychiatric disorders can persist long after detoxification. A protracted (post-acute) withdrawal syndrome can also occur with psychiatric and other symptoms persisting for months after cessation of use. Most dual diagnosis treatment centers specialize mostly on chemical dependency and typically employ interns, drug and alcohol counselors, or recent graduates who may not necessarily have the same competencies that come with years of experience and education. As a result, very few do a good job at properly diagnosing and treating co-occurring psychological disorders. Therefore, clients do not always get to actually work on resolving many of the core issues underlying their addictive behavior.