title: Children's relative age in class and use of medication for ADHD: a Danish Nationwide Study.
abstract: BACKGROUND
  Previous studies from North America and Iceland have shown that the youngest children within a grade are up to twice as likely to be diagnosed and treated for attention-deficit/hyperactivity disorder (ADHD) compared with their older classmates. We aimed to investigate whether younger age in class is associated with an increased probability of being prescribed medication for ADHD among school-aged children in Denmark.
  
  
  METHODS
  We followed all Danish children between 2000 and 2012 from 1st through 6th grade (7-12 years). Among children who started school on their age-assigned grade level, we estimated the prevalence proportion ratio (PPR) of receiving ADHD medication between the youngest children in class (born in October-December) and the oldest in class (born in January-March), specified by grade level, calendar year and gender. As a sensitivity analysis, we added children not on their age-assigned grade level to the main calculations.
  
  
  RESULTS
  We identified 932,032 eligible children for the main analysis, of whom 17.3% were among the youngest and 26.5% among the oldest in class. In total, 1.2% eligible children filled at least one prescription for ADHD medication in 2000-2012. The average PPR over the study period was 1.08 (95% CI, 1.04-1.12) and remained stable across subgroups and sensitivity analyses. Overall, 40% of children born October-December had entered school a year after their age-assigned grade level.
  
  
  CONCLUSIONS
  Contrary to previous study results, we observed almost no relative age effect on medication use for ADHD among children in Denmark. We postulate that this may be due to the high proportion of relatively young children held back by 1 year in the Danish school system and/or a generally low prevalence of ADHD medication use in the country.
authors.affiliation: Clinical Pharmacology, Institute of Public Health, University of Southern Denmark, Odense C, Denmark; Department of Clinical Chemistry & Pharmacology, Odense University Hospital, Odense C, Denmark.
authors.forename: Anton
authors.initials: A
authors.lastname: Pottegård
authors.forename: Jesper
authors.initials: J
authors.lastname: Hallas
authors.lastname: Hernández-Díaz
authors.forename: Helga
authors.initials: H
authors.lastname: Zoëga
mesh_terms: D000367:Age Factors; D001289:Attention Deficit Disorder with Hyperactivity / Q000188:drug therapy* / Q000453:epidemiology*; D002648:Child; D003718:Denmark / Q000453:epidemiology; D005260:Female; D006801:Humans; D008297:Male; D008774:Methylphenidate; D012042:Registries / Q000706:statistics & numerical data*
chemical_list: D008774:Methylphenidate
keywords: Attention-deficit hyperactivity disorder; age factors; central stimulants; children; drug utilization evaluation