The Prevalence of Dual Diagnosis in Substance Dependence

The high prevalence of dual diagnosis between drug use disorders and other mental illnesses does not mean that one caused the other, even if one appeared first. In fact, establishing causality or direction is difficult for several reasons. Diagnosis of a mental disorder may not occur until symptoms have progressed to a specified level (per DSM); however, sub-clinical symptoms may also prompt drug use, and imperfect recollections of when drug use or abuse started can create confusion as to which came first. Still, three scenarios deserve consideration:

1. Drugs of abuse can cause abusers to experience one or more symptoms of another mental illness. The increased risk of psychosis in some marijuana abusers has been offered as evidence for this possibility.
2. Mental illnesses can lead to drug abuse. Individuals with overt, mild, or even sub-clinical mental disorders may abuse drugs as a form of self-medication. For example, the use of tobacco products by patients with schizophrenia is believed to lessen the symptoms of the disease and improve cognition (see “Smoking and Schizophrenia: Self-Medication or Shared Brain Circuitry?”).
3. Both drug use disorders and other mental illnesses are caused by overlapping factors such as underlying brain deficits, genetic vulnerabilities, and/or early exposure to stress or trauma.

REFERENCE:

http://www.drugabuse.gov/publications/comorbidity-addiction-other-mental-illnesses/why-do-drug-use-disorders-often-co-occur-other-mental-illnesses

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