Why do Attention Deficit Hyperactivity Disorder (ADHD) and bipolar disorder often co-occur?

Why ADHD and Bipolar Disorder Frequently Co-Occur

ADHD and bipolar disorder co-occur due to strong shared genetic factors, with first-degree relatives of individuals with either condition showing significantly elevated risk for the other disorder, indicating overlapping familial vulnerability. 1, 2

Epidemiological Evidence of Co-Occurrence

The comorbidity between ADHD and bipolar disorder is substantial and bidirectional:

• Individuals with ADHD have approximately 6-fold increased risk of developing bipolar disorder (OR = 6.06), while those with bipolar disorder have nearly 9-fold increased risk of having ADHD (OR = 8.94). 2
• Approximately 20% of adults with bipolar disorder have comorbid ADHD, and similar rates are seen in borderline personality disorder. 3, 4
• The co-occurrence appears more common in younger children and early-onset cases. 5

Genetic and Familial Mechanisms

The familial clustering provides the strongest evidence for shared etiology:

• First-degree relatives of ADHD patients have nearly 2-fold increased risk of bipolar disorder (OR = 1.94), while first-degree relatives of bipolar patients have 2.7-fold increased risk of ADHD (OR = 2.71). 2
• Offspring of parents with bipolar disorder display early warning symptoms including mood lability, anxiety, attention difficulties, and hyperarousal—symptoms that overlap significantly with ADHD presentation. 1
• Early-onset and highly comorbid cases show even higher degrees of familiality than typical adult-onset bipolar disorder, suggesting genetic loading may predispose to both conditions. 1

Shared Clinical Pathways

Several developmental trajectories link these conditions:

• Premorbid psychiatric problems are common in early-onset bipolar disorder, especially disruptive behavior disorders and behavioral dyscontrol—presentations that overlap with ADHD. 1
• Certain temperamental patterns may presage bipolar disorder, including dysthymic, cyclothymic, or hyperthymic (irritable, driven) temperaments that share features with ADHD. 1
• Approximately 20% of youths with major depression eventually develop manic episodes by adulthood, particularly those with rapid onset, psychomotor retardation, or family history of affective disorders—and depression frequently co-occurs with ADHD. 1

Symptom Overlap and Diagnostic Challenges

The conditions share multiple overlapping symptoms that complicate differentiation:

• Both conditions involve impulsivity and emotional dysregulation, though the quality and pattern differ. 6, 3
• Irritability can be present in both conditions but manifests differently—chronic and trait-like in ADHD versus episodic in bipolar disorder. 6
• Manic-like behaviors in children with ADHD can be commonplace in youth with disruptive behavior problems, leading to diagnostic confusion. 6

Critical Diagnostic Distinction

Bipolar disorder is episodic with periods of normal mood (though not necessarily function), while ADHD symptoms are chronic and trait-like. In patients with comorbid ADHD-BD, ADHD symptoms remain apparent between bipolar episodes. 3

Clinical Implications

The high comorbidity necessitates systematic screening:

• Primary care clinicians should include assessment for coexisting conditions when evaluating children for ADHD, including emotional or behavioral conditions like bipolar disorder. 7
• The validity of bipolar disorder diagnosis in young children remains controversial, with particular caution needed in preschool children where ADHD symptoms may be misinterpreted as mania. 6
• A clear epidemiological overlap and strong familial association advocates for more systematic screening of both conditions when either is present. 2