There is evidence that in adulthood, generalized anxiety disorder (GAD) and attention deficit hyperactivity disorder (ADHD) frequently occur together. Comorbidity for ADHD and GAD was 25–30% in several different adult patient populations. According to the prevalence reported in the literature, 13-51% of children with ADHD have comorbidity with an anxiety disorder. ADHD may have comorbidity with multiple anxiety disorders; Wilens et al. found that 28% of preschool children with ADHD and 33% of school children with ADHD had two or more anxiety disorders.
While both disorders are relatively common, the observed rates of co-occurrence are higher than would be expected on this basis of chance alone. For example, the fidgeting and restlessness of ADHD is similar to the physical restlessness of GAD. It has been suggested that certain forms of anxiety may be manifested as "anxious impulsivity".
Milberger et al. directly examined the overlap of ADHD and GAD in adults and found that 75% of the sample still met criteria for ADHD and 76% of the sample still met criteria for GAD after eliminating symptoms common to both disorders.
The combined presence of ADHD and GAD leads to a more complicated, severely symptomatic presentation of each disorder. In a six-year follow-up of teenagers with ADHD, those subjects with ADHD persisting into adulthood had more anxiety than those with sub-clinical ADHD.
In one large multi-center study (Rösler M, et al., 2010), while there was a significant reduction in ADHD symptoms, there was no difference in change in anxiety symptoms between placebo and methylphenidate. On the other hand, the US Food and Drug Administration (FDA) label warning that stimulant medications may produce increased anxiety. At the same time, a review by Clemow reported that atomoxetine, often used as an alternative to psychostimulants, was not associated with an exacerbation of anxiety. Melegari, M. G., Bruni, O., Sacco, R., Barni, D., Sette, S., & Donfrancesco, R. (2018). Comorbidity of Attention Deficit Hyperactivity Disorder and Generalized Anxiety Disorder in children and adolescents. Psychiatry Research.
Reimherr, F. W., Marchant, B. K., Gift, T. E., & Steans, T. A. (2017). ADHD and Anxiety: Clinical Significance and Treatment Implications. Current Psychiatry Reports, 19(12).
Piñeiro-Dieguez B, et al. Psychiatric comorbidity at the time of diagnosis in adults with ADHD: the CAT study. J Atten Disord. 2016;20(12):1066–75.
Rösler M, et al. Twenty-four-week treatment with extended release methylphenidate improves emotional symptoms in adult ADHD. World J Biol Psychiatry. 2010;11(5):709–18.
Milberger S, et al. Attention deficit hyperactivity disorder and comorbid disorders: issues of overlapping symptoms. Am J Psychiatry. 1995;152(12):1793–9
Clemow DB, et al. A review of the efficacy of atomoxetine in the treatment of attention-deficit hyperactivity disorder in children and adult patients with common comorbidities. Neuropsychiatr Dis Treat. 2017;13:357–71.
Meinzer, M. C., Pettit, J. W., & Viswesvaran, C. (2014). The co-occurrence of attention-deficit/hyperactivity disorder and unipolar depression in children and adolescents: A meta-analytic review. Clinical Psychology Review, 34(8), 595–607.