A dual diagnosis of mental illness and substance abuse is a very common and often underestimated problem. According to the National Alliance on Mental Illness (NAMI), as many as half of all patients with a mental health disorder also have a substance abuse problem. Mental illnesses that are mostly likely to occur with substance abuse include mood disorders, anxiety disorders, personality disorders and eating disorders. Many substances can also produce the symptoms of mental disorders, especially anxiety, depression, psychosis or loss of touch with reality, The dual diagnosis of mental illness and substance abuse leads to more problems for the patient, including physical illness, failure to respond to treatment and relapse than with either condition alone.

The precise reasons why mental disorders and substance abuse problems frequently occur together are not known, but many factors may be involved. Many patients with mental illnesses abuse substances in an attempt to self-medicate. Social factors (e.g., stress, trauma) that impact mental health patients may predispose them to substance abuse. Further, substance abuse is frequently a risk factor for mental illness in itself.

Patients with both mental illness and substance abuse problems are frequently under-diagnosed. In many cases, only one of the conditions is identified. When the full dual-diagnosis is recognized, treatment may be difficult to receive. Many services available to patients with either mental illness or substance abuse problems are not ideally suited to handle both conditions. Because of this, patients are frequently required to move between separate mental health services and substance abuse programs. In many cases, these patients are refused treatment at one facility due to the other condition.