Psychotropic drug prescribing for children and adolescents is frequently off-label and has increased over time and can be controversial. Psychotropic prescribing in two large UK medium secure units for young people has been studied. A total of 89 patients were included, 64% being aged less than 18 years. A total of 137 of 202 (67.8%) of prescriptions were off-label. The most common reasons for a prescription being off-label were the indication (N = 103) and the patient's age (N = 41). The main classes of drugs involved were antipsychotics (N = 59), antiepileptics as mood stabilisers (N = 22), anticholinergics and hyoscine (N = 15) and antidepressants (N = 11). Aggression (N = 48) and post-traumatic stress disorder (N = 30) were the most common off-label indications. Some antidepressant prescriptions were contrary to advice of the Committee on Safety of Medicines (CSM). Meta-analyses or randomised controlled trials supported 27% of off-label prescriptions, with lesser quality studies supporting a further 29.2% and expert opinion 38.7%, whereas for 5.1% no evidence could be found. Prescribers tended to over-estimate the level of evidence from clinical trials or extrapolated from findings in adults. They often quoted their own experience rather than expert sources to justify their prescribing practice. It is important that prescribers are fully aware of the quality of experimental data and the risk-benefit ratio when prescribing off-label for young persons. If the evidence base is limited, it is particularly important to provide information about the risks and benefits of the treatment to the patient/relatives. A second opinion may be helpful. Both target symptoms and side effects should be monitored and regularly reviewed.