Objective To describe the use of medicines and to determine the frequency of off-label use in emergency room paediatric patients. Patients and methods A prospective, observational and descriptive study was carried out in the setting of the paediatric emergency room of a Spanish general hospital. Medicines used by children <14 years prior to their emergency room visit were analysed based on information collected from parents/guardians and relatives for each drug prescription. Off-label use was defined as the utilization of a drug at an indication, dosage, frequency or route of administration that differed from the specifications in the Summary of Product Characteristics or by children outside the authorized age group. Results The patient cohort comprised 462 children, among whom 336 children had been prescribed 667 prescriptions. Of the medicines prescribed, 90% fell into only five 5 Anatomical Therapeutic Chemical Classification System groups. The most frequent active principles were ibuprofen and paracetamol. Of a total of 152 different formulations recorded, no paediatric information was provided for 40 formulations, and one formulation was contraindicated in children. Based on the established criteria, 338 prescriptions were off-label: no paediatric information or contraindication in children were available (82 prescriptions); the drug was used for an indication different from the authorized one (111 prescriptions); drug use was inconsistent with age recommendations (16 prescriptions); drug use was inconsistent with dose/frequency (129 prescriptions). Of the 152 formulations, 107 were occasionally used in an off-label manner. Conclusions Although the mean number of drugs used in children is small, off-label use is frequent. Research efforts should target paediatric studies that allow a rational drug use in children.