AbstractPurposeTo systematically characterize antipsychotic medication coprescribing (‘polypharmacy’) in a large state hospital system.MethodsAll antipsychotic prescriptions written for all adult in‐patients (N=8212) in New York state‐run civil facilities for the year 1999 were identified using the Integrated Research Database (IRDB) created by the Information Systems Division of the Nathan Kline Institute for Psychiatric Research. Antipsychotics were considered to be intentionally coprescribed only when both were prescribed for an overlapping period of 28 days.ResultsCoprescribing of typical, atypical, and depot antipsychotics comprised 31% of antipsychotic prescribing episodes. Medications were usually coprescribed with medications from outside their own antipsychotic class. Patient factors, such as age, diagnosis, and history of prior hospitalization, affected coprescribing rates ( p<0.001 for all indicated variables). Atypical antipsychotic medications were less likely to be given with another antipsychotic than were oral or depot typical medications.ConclusionsCoprescribing of antipsychotic drugs is a common practice in the New York State hospital system. The analysis of large clinical databases can yield valuable information about the kinds of complex pharmacotherapy regimens actually utilized in the treatment of the most severely ill patients. Copyright © 2002 John Wiley & Sons, Ltd.