Attention deficit hyperactivity disorder (ADHD), conduct disorder (CD), and oppositional defiant disorder (ODD) are frequently comorbid. Previous studies suggested that the comorbidity of CD and ODD in ADHD may increase the risk of a further development of mood disorder, but most studies had a small sample size. Using a population-based prospective study design, a large sample composed of 1277 adolescents with ADHD-alone, 46 with ADHD + ODD, 87 with ADHD + CD, and 5640 age/gender-matched controls were enrolled in 2003. These cases were followed to 2010 to identify the cases developing unipolar depressive disorder and bipolar disorder. ADHD + CD groups exhibited a higher prevalence of unipolar depressive disorder (23.0% vs. 13.0% vs. 8.7% vs. 0.7%, p < 0.001) and bipolar disorder (3.4% vs. 2.2% vs. 1.3% vs. 0.2%, p < 0.001) than ADHD + ODD group, ADHD-alone group, and control group. Adolescents with ADHD + CD, those with ADHD + ODD, and those with ADHD-alone had a higher likelihood of developing unipolar depressive disorder (hazard ratio [HR]: 44.34, 95% confidence interval [CI]: 23.95-71.36; HR: 18.76, 95%CI: 7.87-44.71; HR: 13.01, 95%CI: 8.99-18.82) and bipolar disorder (HR: 14.39, 95%CI: 4.00-51.80; HR: 8.32, 95%CI: 1.06-65.32; HR: 5.24, 95%CI: 2.44-11.24) than the controls. Adolescents with ADHD had elevated risks of unipolar depression and bipolar disorder in their later life, and especially, those with ADHD and comorbidity of CD or ODD exhibited the highest risk. Further study would be required to evaluate whether prompt intervention for ADHD and disruptive behavior problems would decrease the risk of developing mood disorder.
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