This paper highlights the inadequacy of existing research for the purposes of evidence-based prn (Latin, pro re nata or 'as needed') medication practice in psychiatric settings and notes the absence of relevant evidence-based clinical practice guidelines and policies, both nationally and internationally. The professional, ethical and legal importance of PRN medication practices is also discussed, and shown to add to the urgency of developing a research agenda which will serve as an adequate basis for good clinical practice. This paper summarises the relevant research and identifies problems that can arise for clinicians involved in the administration of prn medications in mental health settings. It concludes by highlighting key issues which urgently require empirical investigation.