A study published in Health Affairs examined Medicare Prescription Drug Benefit Program (Part D) selection in 2006, finding that enrollees in stand-alone prescription drug plans had higher baseline costs and poorer health than enrollees in Medicare Advantage (MA) prescription plans.  The authors speculate that such patterns could adversely affect Medicare costs in the future.

Riley, G.F., Levy, J.M. & Montgomery, M.A. (2009). Adverse selection in the Medicare prescription drug program. Health Affairs 28(6): 1826-1837. doi: 10.1377/hlthaff.28.6.1826. http://content.healthaffairs.org/cgi/content/abstract/28/6/1826

Authors: Gerald F. Riley, Jesse M. Levy, Melissa A. Montgomery