WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT• Off-label use in children is widespread. New medicines lack marketing authorization for paediatric use, even when they represent significant therapeutic advantages and are intended for treatment of conditions common in children.• Until now no information exists on how off-label use in children develops over time after a significant new medicine is approved for adults and what happens when it is later labelled for one paediatric age group. WHAT THIS STUDY ADDS• Off-label use of a new significant medicine begins in adolescents and extends to younger children with delay. First marketing authorization to adolescents, providing a more child-friendly formulation, results in increase of off-label use in younger children, and has limited effect on total off-label use. AIMTo investigate the evolution of paediatric off-label use after a therapeutically new group of medicines for a common condition becomes available for adults but is labelled for children with a delay of several years. METHODSTriptans were used as a model, because migraine is common in children, and is the only indication for triptans. Data on all triptan prescriptions 1994-2007 were extracted from the nationwide Finnish Prescription Register. Prescriptions for children were compared over time. RESULTS CONCLUSIONSWhen a new significant medicine becomes available in adults, off-label use in children starts slowly but continues to extend to younger children reaching a market size which is little influenced by late appearance of a labelled product. Paediatric treatment remains dominated by off-label use despite labelling of a product in an age appropriate formulation to the most relevant age group.British Journal of Clinical Pharmacology DOI:10.1111DOI:10. /j.1365DOI:10. -2125DOI:10. .2010 Br J Clin Pharmacol / 71:6 / 929-935 / 929