• Simon J C Davies, Martin S Lennard, Parviz Ghahramani, Peter Pratt, Andrea Robertson, and John Potokar.
• Psychopharmacology Unit, University of Bristol, Bristol, UK. simon.davies@bristol.ac.uk
• J. Psychopharmacol. (Oxford). 2007 Mar 1; 21 (2): 153-60.

AbstractMedications are commonly prescribed to psychiatric inpatients on a PRN (pro re nata/as required) basis, allowing drugs to be administered on patient request or at nurses' discretion for psychiatric symptoms, treatment side effects or physical complaints. However, there has been no formal study of the pharmacokinetic implications of PRN prescribing. The objective of the study was to determine the prevalence of PRN drug prescription and administration, and to assess the potential for interactions involving CYP2D6 and CYP3A4 between drugs prescribed and administered to inpatients on psychiatry wards.A cross-sectional survey of prescriptions on general adult and functional elderly psychiatric wards in one city was carried out. Data were recorded from prescription charts of 323 inpatients (236 on general adult and 87 on functional elderly wards). Of 2089 prescriptions, 997 (48%) of prescriptions were on a PRN basis (most commonly benzodiazepines and other hypnotic agents, antipsychotics, analgesics and anticholinergic agents), but only 143 (14%) of these had been administered in the previous 24 hours. One fifth of patients were prescribed drug combinations interacting with CYP2D6 or CYP3A4 of potential clinical importance which included one or more drugs prescribed on a PRN basis.PRN prescribing is common among inpatients in psychiatry, and may lead to cytochrome P450 mediated interactions. Prescribers should be aware of the potential for unpredictability in plasma concentrations, side effects and efficacy which PRN prescribing may cause through these interactions, particularly in old age psychiatry and in treatment of acute psychosis.

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