As previously indicated an association may exist between early sleep problems in infants and toddlers, and a diagnosis of
attention deficit hyperactivity disorder (
ADHD). The aim of this study was to study if this association could be replicated in a complete nationwide cohort of children. Prospective cohort study using national registries. All children born in Norway from January 2004 to December 2010 were included (N = 410,555). Information on
hypnotic drugs dispensed to children 0-3 years of age outside of institutions was collected from the Norwegian Prescription Database and used as a proxy for sleep problems. The outcome
ADHD (ICD-10), as diagnosed by specialists in the Child Mental Health Service, was obtained from the Norwegian Patient Registry. Data were analysed using weighted estimation in Cox regression. The unadjusted weighted hazard ratio (wHR) for a later diagnosis of
ADHD in children dispensed two or more prescriptions for any
hypnotic drug, compared to zero prescriptions, was 2.30 [95% confidence interval (CI) 1.63-3.23] for girls and 1.75 (95% CI 1.48-2.07) for boys. For the
sedative antihistamine trimeprazine the corresponding wHR was 3.71 (95% CI 1.83-7.52) for girls and 2.78 (95% CI 2.04-3.80) for boys. After adjusting for parental
ADHD and parental education the wHR for
trimeprazine users was 2.81 (95% CI 1.34-5.88) for girls and 2.33 (95% CI 1.70-3.20) for boys. Infants and toddlers who were dispensed
hypnotics had an increased risk of
ADHD at school age. This association was most pronounced with the use of
trimeprazine, a
drug traditionally prescribed to toddlers for sleep problems in Norway. After adjusting for parental
ADHD and educational level the risk for
ADHD among the
trimeprazine users was still more than twice the risk among controls.