Common etiological factors of attention-deficit/hyperactivity disorder and suicidal behavior: a population-based study in Sweden.

IMPORTANCE The prevention of suicidal behavior is one of the most important tasks for mental health clinicians. Although a few studies have indicated an increased risk of suicidal behavior among individuals with attention-deficit/hyperactivity disorder, the development of more effective ways of identifying and modifying the risk is hampered by our limited understanding of the underlying mechanisms for this association. OBJECTIVE To explore whether attention-deficit/hyperactivity disorder and suicidal behavior share genetic and environmental risk factors. DESIGN, SETTING, AND PARTICIPANTS Matched cohort design across different levels of family relatedness recorded from January 1, 1987, to December 31, 2009. We identified 51 707 patients with attention-deficit/hyperactivity disorder (through patient and prescribed drug registers) in Sweden and their relatives by linking longitudinal population-based registers. Control participants were matched 1:5 on sex and birth year. MAIN OUTCOMES AND MEASURES Any record of suicide attempt or completed suicide defined by discharge diagnoses of the International Classification of Diseases. RESULTS Individuals with attention-deficit/hyperactivity disorder (probands) had increased risks of attempted and completed suicide, even after adjusting for comorbid psychiatric disorders (odds ratio [OR] = 3.62 [95% CI, 3.29-3.98] and 5.91 [95% CI, 2.45-14.27], respectively). The highest familial risk was observed among first-degree relatives (attempted suicide: OR = 2.42 [95% CI, 2.36-2.49] among parents of probands with ADHD and OR = 2.28 [95% CI, 2.17-2.40] among full siblings of probands with ADHD; completed suicide: OR = 2.24 [95% CI, 2.06-2.43] and OR = 2.23 [1.83-2.73], respectively), whereas the risk was considerably lower among more genetically distant relatives (attempted suicide: OR = 1.59 [95% CI, 1.47-1.73] among maternal half siblings, OR = 1.57 [95% CI, 1.45-1.70] among paternal half siblings, and OR = 1.39 [95% CI, 1.35-1.43] among cousins; completed suicide: OR = 1.51 [95% CI, 1.08-2.10], OR = 2.02 [95% CI, 1.47-2.79], and OR = 1.51 [95% CI, 1.36-1.67], respectively). These familial aggregation patterns remained similar across sex, after excluding relatives with attention-deficit/hyperactivity disorder and probands with suicidal behavior, and after excluding probands and relatives with severe comorbid disorders. CONCLUSIONS AND RELEVANCE Attention-deficit/hyperactivity disorder is associated with an increased risk of both attempted and completed suicide. The pattern of familial risks across different levels of relatedness suggests that shared genetic factors are important for this association. This is an important first step toward identifying the underlying mechanisms for the risk of suicidal behavior in patients with attention-deficit/hyperactivity disorder and suggests that individuals with attention-deficit/hyperactivity disorder and their family members are important targets for suicide prevention and treatment.