The Medicare Prescription Drug, Improvement, and Modernization Act of 2003 (MMA) added prescription drug coverage to Medicare (Medicare Part D), beginning on January 1, 2006. Medicare's elderly and disabled beneficiaries can enroll in private plans that contract with Medicare to provide the drug benefit. People on Medicare can obtain the Medicare drug benefit through two types of private plans. Beneficiaries can join a prescription drug plan (PDP) for drug coverage only and get their other Medicare benefits through the fee-for-service program, as the majority of people with Medicare currently do. Alternatively they can join a Medicare Advantage (MA) plan, such as an HMO or regional PPO, that covers drugs and all other Medicare benefits. Private plans can operate in one or more of 34 PDP regions and 26 MA regions nationwide.
For more information, visit Kaiser's Resources on the Medicare Drug Benefit, available at http://www.kff.org/medicare/rxdrugbenefit.cfm.

Mathematica Policy Research analysis of CMS March 2009 Monthly PDP Enrollment by State/County/Contract file. For more information, visit Kaiser's Medicare Health and Prescription Drug Plan Tracker at http://healthplantracker.kff.org/topicresults.jsp?i=63&rt=2.

PDPs: Prescription Drug Plans.
Penetration: is the number of enrollees in Prescription Drug Plans divided by the number of Medicare beneficiaries.