A pervasive phenomenon in developing countries is that self-prescribed medications are purchased from drug vendors without professional supervision. In this article we develop a model of self-medicating behavior of a utility-maximizing consumer who balances the benefits and risks of self-medication. The empirical investigation focuses on the role of income and health insurance on the use of self-medication. Our data are from the World Bank's Living Standards Measurement Survey of Vietnam, 1997-1998. The results show that self-medication is an inferior good at high income levels and a normal good at low income levels, and it shows a strong and robust negative insurance effect. Copyright © 2003 John Wiley & Sons, Ltd.