What is comorbidity? Psychiatric comorbidity refers to the occurrence of 2 or more mental or substance use disorders within a certain period. Research shows that comorbidity of substance use and other psychiatric disorders is common and often worsens the prognosis for each disorder; increases the personal disease burden; increases the frequency of hospitalization (revolving-door effect); increases the risk of other medical and psychosocial consequences, including suicide; and makes treatment more difficult. What follows is a brief review of the epidemiology of comorbidity of illicit drug use disorders and major mental disorders, summarizing the results of several large epidemiologic studies conducted in the United States over the past 2 decades.
In order to maximize the ability to compare findings and draw conclusions across these studies, this review focuses on comorbidity of illicit drug use disorders and several mental disorders that have been consistently associated with them: antisocial personality disorder (ASPD), mood disorders, and anxiety disorders. Following a brief review of the published literature, we offer some interpretations and clinical implications of such comorbidity. Readers are directed elsewhere for reviews of the comorbidity of psychiatric disorders with alcohol use disorders in adulthood,1 nicotine dependence in adulthood,2,3 and substance use disorders in childhood and adolescence.4
Major US epidemiologic studies
Several large epidemiologic studies have directly examined the comorbidity of mental and illicit drug use disorders among adults in the United States. Major studies with published comparable data on this topic include the Epidemiologic Catchment Area(ECA) study, the National Comorbidity Survey (NCS), and the National Epidemio- logic Survey on Alcohol and Related Conditions (NESARC). These studies survey individuals from the general population, apply advanced sampling strategies to maximize generalizability and case yield, use standardized assessment instruments, define psychiatric disorders according to DSM, and administer structured interview schedules by carefully trained interviewers. A snapshot of the methods of these studies is presented in Table 1.
From a statistical perspective, the probability of co-occurrence of mental and drug use disorders is indicated most commonly by the odds ratio (OR), which is an estimate of the likelihood that a certain event is the same for 2 groups. The OR is a convenient and widely used index of comorbidity that is readily interpretable and comparable across different epidemiologic studies.5 An OR of 1.0 means that the event is equally likely to occur in both groups of individuals; an OR exceeding 1.0 means that the event is more likely in the first group; an OR that is less than 1.0 means that the event is less likely in the first group.
Three major epidemiologic studies of psychiatric comorbidity of illicit drug use disorders among adults in the United States
|Epidemiologic Catchment Area study|
|DSM-III; DIS39; N = 19,460; 5-site community sample; aged > 18 years;
fielded 1980 - 1984
|National Comorbidity Survey|
|DSM-III-R; N = 88,098; nationally representative sample; aged 15 - 64 years;
fielded 1990 - 1992
|National Epidemiologic Survey on Alcohol and Related Conditions|
|DSM-IV; AUDADIS40; N = 43,093; nationally representative sample; aged > 18 years;
fielded 2001 - 2002
DIS, Diagnostic Interview Schedule; AUDADIS, Alcohol Use Disorder and Associated Disabilities Interview Schedule.
Epidemiologic studies report extensive lifetime psychiatric comorbidity among people who have drug use disorders. Some of the most extensive and consistent findings have been reported among drug use disorders and ASPD, mood disorders, and anxiety disorders. Table 2 presents the associations between drug use disorders (separately for drug abuse and drug dependence) and these mental disorders according to published reports from the ECA study, NCS, and NESARC.6-9