Dual diagnosis refers to people who have been diagnosed with a major mental illness along with addiction. The problem is that substance abuse, which has components of mental illness, often masks and/or aggravates other psychological symptoms. Conversely, those with psychological symptoms may self-medicate with drugs and alcohol. The result can be confusion over the needed treatment resulting in relapse and other major problems.
As a general rule, treatment for mental illness is close to impossible if the mentally ill addict is still abusing the drug of choice. On the other hand, the addict may find it extremely difficult to actually stop using until at least some of the psychiatric symptoms are relieved. Ideally, both conditions are recognized and treated together. In practice one problem is often recognized before (sometimes long before) the other.
There are treatment centers for the dually addicted that address both the mental illness and the addiction. There are also 12 Step meetings and other support groups that are aimed at dual addiction.