What is dual diagnosis?
This may be a primary diagnosis of major mental illness with a subsequent diagnosis of substance use which affects mental health adversely such as cannabis use on top of schizophrenia. Conversely, there may be primary diagnosis of drug dependence which leads to mental illness such as is found with chronic use of stimulants like amphetamine or cocaine. In the US it is also acknowledged that there can be two diagnoses co-existing independently or due to the same cause.
Because in many cases the signs and symptoms of mental illness can be mimicked by the intoxicated or withdrawing state of substance use, the major diagnostic issue is making an accurate diagnosis of the primary problem. The consensus seems to be that a period of three to six weeks of abstinence is needed for a fully accurate diagnosis to be made. Equally important is that a careful and full history of signs and symptoms from the client and significant others will make a crucial difference is sorting out what might otherwise be a confused diagnosis.
The key issue of correct diagnosis was highlighted in a recent Lancet article which claimed that too many people were being rapidly diagnosed as schizophrenic without consideration of the role of drugs and alcohol in the presenting symptoms.