This article analyzes the shift in prescription drug spending from Medicaid to Medicare as a result of Medicare Part D.  Among other findings, the authors note that Medicaid prescription drug spending declined by nearly 50 percent in 2006 and that the policy shift impacted the classes of drugs prescribed through both programs. 

Bruen, B. K., & Miller, L. M. (2008). Changes in Medicaid prescription volume and use in the wake of Medicare Part D implementation. Health Affairs, 27(1): 196-202. DOI: 10.1377/hlthaff.27.1.196. http://content.healthaffairs.org/cgi/content/abstract/27/1/196