The administration of PRN medication by mental health nurses is an important, yet poorly explored aspect of psychiatric inpatient care. An examination of nurses' reasons for administering PRN medication is essential in ensuring its appropriate and effective use. Data were gathered from the drug charts of 44 inpatients on two acute psychiatric wards. Most PRN medication was given orally and the most frequently administered drugs were procyclidine, lorazepam, ibuprofen, diazepam and droperidol. The main reason for administering PRN medication was because patients had 'requested' it. Results were broadly consistent with previous research. It is recommended that nurses should give clear and specific reasons for administering PRN medication based on a valid assessment. Implications for clinical practice and further research are also discussed.