Little is known about the extent of the use of prn psychotropic medication in psychiatric inpatient units. A survey of the prn prescription and administration of psychotropic drugs in a psychiatric teaching hospital revealed that a large number of inpatients were prescribed and administered such drugs on a prn basis. Although 50% of the prescriptions were never administered, only 25% were actively discontinued by physicians. A diagnosis of personality disorder was the factor most frequently associated with the rate of prn prescriptions and of administrations. A large number of prn prescriptions had no instructions for indications, minimum time spacing between doses or maximum daily dosage. It is suggested that hospitals monitor the prn use of psychotropic medications in their inpatient units, and explore the reasons for such use. Psychotropic drug use on a prn basis should preferably be reserved for emergencies, and the instructions of prn prescriptions should be clear and detailed.