This integrative review explores the current pro re nata (PRN) medication practice in acute adult mental health settings. PRN medication is commonly used in acute mental health settings but there is lack of evidence of effectiveness of this practice. PRN medications have a number of adverse effects and increase the risk of morbidity in patients with a mental illness. Articles were identified from MEDLINE, CINAHL, Scopus, PubMed, PsycINFO, and Web of Science database. The STROBE critical appraisal tool was used to evaluate the quality of evidence, and inductive thematic analysis was used to extract main themes. Five themes regarding prescription practices, poor documentation, reasons to administer, medication misuse, and insufficient use of non‐pharmacological interventions emerged among the 12 eligible articles. The study identified PRN medication practice gaps in adult mental health settings included insufficient documentation practice, underuse of therapeutic non‐pharmacological interventions, and significant variability in PRN medication practice across the mental health professionals due to different levels of knowledge and experience.