Several observations are apparent from the pattern of results in Table 2. First, the 3 studies show that drug use disorders are highly comorbid with ASPD, mood disorders, and anxiety disorders. The ORs are nearly universally greater than 1.0 and are statistically significant.
Second, the risk of ASPD, mood disorders, and anxiety disorders is notably greater for individuals with drug dependence than for those with drug abuse. The association between drug dependence and ASPD, for example, is 3 to 5 times greater than the association between drug abuse and ASPD. Regarding internalizing disorders, the association between drug dependence and any mood or anxiety disorder is 2 to 3 times greater than the association between drug abuse and any mood or anxiety disorder. Table 2 also shows higher ORs for drug dependence than for drug abuse across nearly all subtypes of mood and anxiety disorders, although it is particularly evident for bipolar disorder, mania, and panic with agoraphobia. Other findings from the NESARC show greater associations for dependence than for abuse across 8 specific drug use disorders.6,10
Third, ASPD is the mental disorder most strongly associated with drug abuse or dependence. Across the 3 studies, the ORs between drug abuse or dependence and ASPD are consistently much higher (range, 2.5 to 16.7) than between drug abuse or dependence and any mood disorder (range, 1.7 to 7.1) or any anxiety disorder (range, 1.4 to 4.9).
Fourth, there is mixed evidence that mood disorders are more highly associated with drug use disorders than are anxiety disorders. The ECA study and the NESARC report ORs between drug use disorders and any mood disorder that are greater than the corresponding ORs for any anxiety disorder. Recent findings from the NESARC also show greater associations for mood disorders than for anxiety disorders across 8 specific drug use disorders, with the greatest associations between each of the specific drug use disorders and comorbid mood and anxiety disorder, followed by pure mood disorder, and then pure anxiety disorder.6 In contrast to the findings from the ECA study and the NESARC, the NCS reported associations between drug dependence and any anxiety disorder that exceed that for any mood disorder, whereas the OR for drug abuse and any mood disorder exceeded that for any anxiety disorder.
Table 3 presents additional findings from the NESARC indicating that certain pairwise associations between specific mental disorders and specific drug use disorders are greater than others.6,10 This level of matching is made possible for the first time by the large sample size of the NESARC study. Results show that although nearly every specific mental disorder is positively and significantly associated with each specific drug use disorder, the magnitude of the associations is inversely related to the prevalence of the specific drug use disorder. That is, marijuana use disorder is by far the most prevalent drug use disorder, followed by cocaine use disorder, yet these drug use disorders generally show the weakest associations with mood, anxiety, and antisocial personality disorders. Conversely, low-prevalence drug use disorders (eg, sedative, opioid, and tranquilizer abuse or dependence) are more strongly associated with those mental disorders. Regarding the specific mental disorders, the largest ORs are seen for mania and panic disorder with agoraphobia. These mental disorders are most strongly associated with several less prevalent drug use disorders involving sedatives, tranquilizers, and opiates. The ORs for these associations range from 6.2 to 8.4.
Interpretations of key findings
Across major epidemiologic studies of adults living in the United States, there is a strong consensus that drug use disorders co-occur at higher-than-chance levels with ASPD, mood disorders, and anxiety disorders. The accord across the ECA study, NCS, and NESARC is particularly impressive given the nontrivial methodological differences across studies presented in Table 1.
The extensive comorbidity among these psychiatric disorders can be interpreted in several ways, although more fine-grained research is needed to understand the complexity of these co-occurring disorders. First, comorbid disorders are independent disorders with distinct courses that co-occur either by chance alone or because of methodologic issues that overestimate comorbidity. Large epidemiologic studies can be especially prone to overestimation if they focus only on lifetime diagnoses in mixed-age samples and use assessments that include subthreshold conditions as diagnoses.5,11 Yet, it is noteworthy that significantly elevated ORs are reported throughout the literature despite important methodological variations, including epidemiologic sample, assessment tools, diagnostic system, and time for estimating comorbidity (eg, lifetime, past year, past month).