Use of Antidepressants Decreased After Initiation of ADHD Treatment in Adults—A Finnish Nationwide Register Study Describing Use of ADHD and Non‐ ADHD

ABSTRACT Introduction ADHD is often associated with comorbid psychiatric conditions. Differential diagnosis between other conditions and ADHD is not always clear, and patients are sometimes initially treated for another disorder instead. ADHD diagnosis and appropriate ADHD treatment potentially reduce the need for medication of the other disorder. Reaching high adherence to and persistence with ADHD medication is challenging. This nationwide cohort study aimed to describe not only ADHD medication use but also the use of other drugs in ADHD patients compared to controls. Methods Nationwide care and prescription registers were used to identify incident ADHD patients of any age between 2015 and 2020. Four controls were matched to each ADHD patient by age, gender, and residence. Analyses included data from 1.1.2010 to 31.12.2021. Results Study cohort included 66,146 ADHD patients and 256,270 controls, with a total follow‐up of 1,123,412 years. Sustained and extended‐release methylphenidate were the two most commonly used first‐line ADHD drugs across all age groups. Simultaneous use of different ADHD drugs was rare. Primary adherence was very high, with 95% of the patients purchasing their prescribed medication in general and 80% doing so within 10 days. Persistence with medication was the highest among the youngest patients. A decrease in purchases was observed during the summer holidays in school‐age children and adolescents. In adults, antidepressant use often preceded ADHD diagnosis and decreased after ADHD treatment initiation, unlike in controls at the same time. In young children, antibiotics and anti‐inflammatory drug use was higher in ADHD patients than in controls, especially before ADHD identification. Conclusion As far as we know, this is the first study to describe changes in the use of non‐ADHD medications in relation to ADHD identification. In adults, antidepressant use decreased after ADHD treatment initiation, and in children, antibiotic and anti‐inflammatory use showed more prominent decrease compared to controls of the same age. The data indicated high primary adherence to ADHD medication, and the youngest children remained on continuous ADHD medication the longest. The effect of summer holidays was visible in the purchase data.