Purpose – Use of off-license medicines in forensic mental health settings is common and unlicensed drugs are sometimes prescribed. Despite their responsibility for administering medicines little is known about how mental health nurses view these practices. The paper aims to discuss these issues. Design/methodology/approach – In total, 50 mental health nurses working in low and medium secure adolescent and adult mental health wards were presented with a clinical vignette about administration of unlicensed and off-license medicines. Semi-structured interviews about their likely clinical response to, and feelings about, this practice were conducted. Interview data were subject to a thematic analysis. Findings – Analysis revealed six themes: status of unlicensed/off-label medicines; legality of administering unlicensed medicines; professional standards around administering unlicensed medicines; finding out more about unlicensed medicines; trusting medical colleagues; and decision making in uncertain cases. Practical implications – Forensic mental health nurses take a pragmatic approach to the practice of administering unlicensed medicines and most are aware of their professional responsibilities. Originality/value – This study provides the first evidence to inform the development of training for forensic mental health nurses about an issue that is common in forensic mental health practice.