Variations in Attention-deficit/hyperactivity disorder medication prescription rates during childhood depending on month of birth

Introduction: Previous studies have reported that there is a seasonal variation in the prevalence of Attention- deficit/hyperactivity disorder (ADHD) diagnosis and medication in among other North America and Europe. The reason for this has suggested being relative immaturity.Aim: To investigate, in the county of Värmland, if children born late in the year are more likely to have medical treatment of ADHD compared to children born early in the year.Material and Methods: a population-based study including all children in the county of Värmland, Sweden, that were born between 2000 – 2011. Patients defined as children with ongoing prescription of ADHD medication and without intellectual disability or severe motor impairment by January 2018. Risk ratio for receiving ADHD medication during different quarters of the year were calculated with reference data from Statistics Sweden (SCB) on the total number of children registered in the present county.Results: The study included 27 950 children and 999 children of them had ADHD medication. The prevalence of ADHD medication had a trend to rise although the year. Risk ratios (95 % CI) for ADHD medication were 1.42 (1.15 – 1.75) in boys and 1.39 (0.98 – 1.92) in girls when comparing the fourth to the first quarter of the year.Conclusions: Children, especially boys, born late in the year have higher rates of ADHD medication. This could lead to that children born early in the year is diagnosed later. The study cannot enounce a cause for this, but relative immaturity is a possibility