RESUMEN
The case report describes the sudden emerge of psychotic symptoms of a 13-year old boy, suffering from attention deficit hyperactivity disorder (ADHD), while he was treated with a modified--release formulation of methylphenidate. The question of the attending physician, whether the symptoms indicate an early beginning of schizophrenia or whether the symptoms can be considered as an overdose phenomenon of methylphenidate should be discussed. The knowledge of the pharmacokinetics of methylphenidate as modified--release formulation and the serum concentration values of methylphenidate and its metabolite provide information for interpretation of the case reported, with regard to the purpose of therapeutic drug monitoring (TDM) for methylphenidate. Finally we conclude that the ingestion of an unintended overdose of methylphenidate may have caused the boy's psychotic symptoms and that routine therapeutic serum concentrations of methylphenidate have to be correlated to the point of time when the blood was drawn in relation to the intake of methylphenidate.