This article explores the effect of prescription drug copayments on services and drug utilization in Medicaid.  The article finds that copayments are associated with reductions in drug use for clinically important drug classes and notes that drug utilization patterns vary among chronically ill patients by drug class.   

Hartung, D. M., Carlson, M. J., Kraemer, D. F., Haxby, D. G., Ketchum, K. L., & Greenlick, M. R. (2008). Impact of a Medicaid copayment policy on prescription drug and health services utilization in a fee-for-service Medicaid population. Medical Care, 46(6): 565-72. http://journals.lww.com/lww-medicalcare/pages/articleviewer.aspx?year=2008&issue=06000&article=00003&type=abstract

Authors: Daniel M. Hartung, Matthew J. Carlson, Dale F. Kraemer, Dean G. Haxby, Kathy L. Ketchum, Merwyn R. Greenlick