Objective. The aim of this study was to investigate the use of "as needed" (pro re nata or prn) psychotropic medication in a child and adolescent psychiatric inpatient population. The study was carried out on the psychiatry ward of a paediatric teaching hospital in Paris, France. Methods. A prospective analysis of prn psychotropic drug prescriptions and administrations was conducted for all patients hospitalised over a period of 4 months. The study group consisted of 187 patients. Results. In total, 93 prn prescriptions were written, for 27% of the patients (51) but only 14% (26) received a total of 76 administrations. Antipsychotic drugs accounted for 54% of the prescriptions, anxiolytics for 33%, antiepileptic drugs for 8%, antiparkinsonian drugs for 4% and hypnotic drugs for 1%. Anxiety was the reason given for 67% of the prn administrations, with hydroxyzine used in 69% of these cases. Disruptive behaviour accounted for 22% of prn administrations, with antipsychotic drugs accounting for 88% of these administrations. Insomnia accounted for 8% of prn administrations, and antipsychotic drug-induced dystonia accounted for 3% of such administrations. Conclusion. Controlled studies are required to assess the efficacy and safety of prn medication and the conditions in which its use is indicated.