SUDs are frequently comorbid with other mental illnesses including internalizing disorders, such as depression and anxiety, and externalizing disorders, such as conduct disorder (CD), antisocial personality disorder, borderline personality disorder, and attention-deficit/hyperactivity disorder (ADHD).24,25,31 Twin studies indicate shared genetic influences between SUDs and externalizing disorders.27,28,32 Longitudinal studies have shown that CD and ADHD are important risk factors for addiction.33 In contrast, twin studies have failed to reveal a shared genetic vulnerability between addiction and internalizing disorder, and it has been suggested that anxiety and depression are more commonly a consequence rather than a risk factor for addiction, being related to neuroadaptation and with drawal.34,35 However, longitudinal studies have shown that some anxiety disorders and anxiety-related personality traits such as panic disorder, social phobia, and increased harm avoidance predict subsequent alcohol problems in adoles cents and young adults.36
