Attention-deficit/hyperactivity disorder (ADHD) treatment rates in adults are low, possibly owing to the discontinuation of care during the transition from adolescence to adulthood. To improve the understanding of treatment patterns during this transition, pharmacotherapy use was characterized in patients with ADHD aged 17–21 years. This retrospective claims analysis of the IBM® MarketScan® Commercial Databases included patients aged 16–21 years with ≥2 ADHD diagnoses between 1/1/2008 and 12/31/2017 (≥1 diagnosis by age 17) who were continuously enrolled and prescribed ADHD medication for ≥6 months at age 17. Pharmacotherapy use, including no treatment (no drug supply) and disrupted treatment ( < 6 months of drug supply across drug classes) was assessed longitudinally starting at age 17 using Wilcoxon signed-rank tests and McNemar tests. Across 10,292 patients, the percentage of patients receiving pharmacotherapy significantly decreased (P < 0.001) from age 16–17 years (100%) to 20–21 years (68.8%). The median time to pharmacotherapy discontinuation was 2.94 years. As patients aged, decreases in the percentage of those using long-acting amphetamines (17 years, 46.0%; 21 years, 17.8%) or long-acting methylphenidates (17 years, 28.6%; 21 years, 7.8%) were observed. At age 21 years, 42.8% of patients were receiving no treatment, and 17.9% had disrupted treatment. After transitioning to disrupted/no treatment, low percentages of patients returned to pharmacotherapy (no treatment: 2.7%%–3.8% per year; disrupted treatment: 15.7%–21.5% per year). Patients rarely reinitiated pharmacotherapy after its disruption or discontinuation, emphasizing the need to manage ADHD treatment as patients transition from adolescence to adulthood.