Prevalence and Incidence of Attention Deficit/Hyperactivity Disorder in Denmark. A National Register-Based Open Cohort Study.

INTRODUCTION

Rises in prevalence and incidence of the neurodevelopmental disorder Attention-deficit/Hyperactivity Disorder (ADHD) have been reported globally and estimated in Denmark previously. Nevertheless, differences in methodology hinder an assessment of the temporal evolution of ADHD based on published literature. The aim of this study was to consistently calculate the yearly prevalence and incidence proportions of ADHD and the use of ADHD medication in Denmark from 2000 to 2022.

METHODS

We conducted an open cohort study using register data covering the entire Danish population. We defined ADHD as either having a hospital diagnosis of ADHD (ICD-10: F90.0, F90.1, F90.8, or F98.8), or having redeemed a prescription for ADHD medication.

RESULTS

Of the included 7,748,837 persons, 2.52% had ADHD, of whom 58.33% were males. The prevalence of ADHD has increased from 0.10% in 2000 to 3.03% in 2022. Specifically for the 18-27-year-olds, the prevalence was 8.16% for males and 6.12% for females in the year 2022. For the incidence, the same age group peaked within the females in 2022 at 1.10%, whereas the male incidence was consistently highest in the 6-18-year-olds, peaking in 2022 at 0.94%. Total use of ADHD medication in the population also increased. Methylphenidate was used by 97.58% of the ADHD medicated patients in 2000 and 70.60% in 2022. Lisdexamphetamine was the second most common in 2022, where 24.03% redeemed a prescription.

LIMITATIONS

Incidence and prevalence were calculated specifically for the Danish population and may not be directly generalizable to other countries.

CONCLUSIONS

The prevalence of ADHD has consistently increased throughout the past two decades. The main contribution to this increase is the ADHD incidence in females, especially young females in the most recent years 2020-2022. Methylphenidate was consistently the most common ADHD drug prescribed, though lisdexamphetamine use has increased in recent years.