Pro re nata (PRN; 'as needed') medication is an archetypal mainstay for managing acute psychiatric inpatient symptoms and behaviours. Psychiatric and mental health nursing practices have circumnavigated the development of a uniform medical-ethical standard for the administration of PRN psychotropic medication. This paper examines the evidence for administration of PRN psychotropic medications and, in the context of evidence-based best practice, current mental health policy and professional ethics, proposes a standardized Australian PRN administration protocol. The procedures and circumstances leading to a nurse administering psychotropic PRN medication are divided into five simple steps, namely (i) medical prescription; (ii) nurse evaluation of patient indications for an intervention; (iii) nurse consideration of therapeutic options; (iv) obtaining patient informed consent; and (v) documentation of outcomes of PRN administration. The literature associated with each step is reviewed, along with national and international professional ethics, guidelines and patient rights documents pertaining to the care of mental health patients. Recommendations for best-practise care are discussed for each step. There is a lacuna of published evidence supporting the use of PRN medications in psychiatric inpatients. Yet there is published evidence that PRN medications are associated with increased risks of morbidity, inappropriate use, may result in above-recommended dosages or polypharmacy, and complicate the assessment of efficacy of regular scheduled medicines. Alternative non-pharmacological treatment options to PRN medication are effective and associated with fewer side-effects. There are no national explicit standards, operational criteria or quality assurance for the use of PRN medication in inpatient psychiatric units. Contemporary PRN practices are largely unregulated and driven by essentially anecdotal evidence, leaving the clinicians and the service open to claims of poor accountability and misuse (intentional and unintentional) of psychotropic medications. Development of best practice guidelines for the use of PRN administration is essential.