Abstract
IMPORTANCE: Autistic adults experience suboptimal health outcomes. Likewise, attention-deficit/hyperactivity disorder (ADHD) is linked to poor health outcomes in the general population, yet little is known about co-occurring ADHD and its links with poor health outcomes among autistic adults. OBJECTIVE: To determine the prevalence of ADHD and ADHD medication prescriptions and to examine associations between health outcomes and both ADHD co-occurrence and ADHD medication prescriptions among autistic adults. DESIGN, SETTING, AND PARTICIPANTS: Population-based cohort study of US Medicaid-enrolled adults aged 18 years and older, including autistic adults with and without co-occurring intellectual disability (ID), adults with ID without autism, and adults from a random sample of Medicaid-enrolled individuals from 2008 to 2019. Analyses were completed September 2023 to September 2024. EXPOSURES: Autism, ADHD, and ID using validated algorithms from the Chronic Conditions Warehouse. MAIN OUTCOMES AND MEASURES: Prevalence rates of co-occurring ADHD, ADHD medication prescriptions, substance use, cardiovascular conditions, injury in autism without ID, ID without autism, autism with ID, and the general population. RESULTS: The study included 3 506 661 patients (mean [SD] age, 33.5 [15.6] years; 1 854 892 [52.9%] female; 702 694 [20.0%] Black, 587 048 [16.7%] Hispanic, and 1 786 703 [60.0%] White). Compared with the general Medicaid-enrolled population (1 846 102 patients), which had 49 523 patients (2.7%) with co-occurring ADHD diagnoses, this study found elevated rates of ADHD in autism without ID (280 195 patients [26.7%]; prevalence ratio [PR], 5.1; 95% CI, 4.4-5.9), autism with ID (261 061 patients [40.2%]; PR, 6.8; 95% CI, 6.0-7.7), and ID without autism (1 119 303 patients [19.0%]; PR, 4.4; 95% CI, 4.0-5.0). A total of 117 704 adults (26.6%) with ADHD received an ADHD medication prescription. The study identified that rates of substance use were higher among clinical groups with co-occurring ADHD. For example, 9886 of 74 675 autistic adults with ADHD (13.2%) had substance use disorder, compared with 11 782 of 205 520 autistic adults with no ADHD (5.7%). ADHD diagnoses were associated with higher rates of all health outcomes, regardless of group. Substance use was higher in both ID groups with ADHD diagnoses and ADHD medications, whereas injury and cardiovascular condition rates were lower in all ADHD groups who received ADHD medications. CONCLUSIONS AND RELEVANCE: In this cohort study of Medicaid-enrolled adults, autistic adults experienced high rates of co-occurring ADHD and were more likely to receive ADHD medication prescriptions than adults in the general population. Negative health outcome rates are higher among autistic people with co-occurring ADHD, although ADHD medication prescriptions are associated with lower rates of negative health outcomes. Treating ADHD may improve health among autistic adults.