Objective-to examine changes in the prevalence of antipsychotic medication use and the characteristics of antipsychotic users in the US population.Methods-We use data from the Medical Expenditure Panel Survey from 1996/97 and 2004/05 to examine the rate of first and second-generation antipsychotic medication use and changes in the characteristics of users of all ages. We examine trends in the level of use by antipsychotic users both in terms of daily dose units and number of prescriptions.Results-The rate of antipsychotic use has increased substantially between 1996/97 and 2004/05, but the average dose measured both by daily dose units and number of prescriptions has remained constant. The rapid diffusion of antipsychotic medications occurred not among individuals with schizophrenia, but rather includes substantial growth in off-label users and newer on-label conditions. Demographic, financial, and insurance characteristics of users have remained fairly constant, with few exceptions. The average age of antipsychotic users has declined from 1996-2005 as more children are using these medications. The gender, racial, ethnic, and insurance composition of users has been fairly stable over time, however.Conclusions-The rapid diffusion of second-generation antipsychotic medications was achieved by large increases in the rate of use in certain subpopulations, most notably children. Increasing understanding of the marginal efficacy and side-effect risks of newer and more expensive antipsychotic agents, even when prescribed as indicated, suggests that the dramatic increase in use warrants careful attention.