Medicare Part D went into effect on January 1, 2006. Anyone with Part A or B is eligible for Part D. It was made possible by the passage of the Medicare Modernization Act. In order to receive this benefit, a person with Medicare must enroll in a stand-alone Prescription Drug Plan (PDP) or Medicare Advantage plan with prescription drug coverage (MA-PD). These plans are approved and regulated by the Medicare program, but are actually designed and administered by private health insurance companies. Unlike Original Medicare (Part A and B), Part D coverage is not standardized. Plans choose which drugs (or even classes of drugs) they wish to cover, at what level (or tier) they wish to cover it, and are free to choose not to cover some drugs at all. The exception to this is drugs that Medicare specifically excludes from coverage, including but not limited to benzodiazepines, cough suppressant and barbiturates.[41][42] Plans that cover excluded drugs are not allowed to pass those costs on to Medicare, and plans are required to repay CMS if they are found to have billed Medicare in these cases.[43]

It should be noted again for beneficiaries who are dual-eligible (Medicare and Medicaid eligible) Medicaid may pay for drugs not covered by part D of Medicare, such as benzodiazepines, and other restricted controlled substances.

The Patient Protection and Affordable Care Act ("ACA") of 2010 made a number of changes to the Medicare program. Several provisions of the law were designed to reduce the cost of Medicare. Congress reduced payments to privately managed Medicare Advantage plans to align more closely with rates paid for comparable care under traditional Medicare. Congress also slightly reduced annual increases in payments to physicians and to hospitals that serve a disproportionate share of low-income patients. Along with other minor adjustments, these changes reduced Medicare's projected cost over the next decade by $455 billion.

Additionally, the ACA created the Independent Payment Advisory Board ("IPAB"), which will be empowered to submit legislative proposals to reduce the cost of Medicare if the program's per-capita spending grows faster than per-capita GDP plus one percent. While the IPAB would be barred from rationing care, raising revenue, changing benefits or eligibility, increasing cost sharing, or cutting payments to hospitals, its creation has been one of the more controversial aspects of health reform.

The ACA also made some changes to Medicare enrollee's' benefits. By 2020, it will close the so-called "donut hole" between Part D plans' coverage limits and the catastrophic cap on out-of-pocket spending, reducing a Part D enrollee's' exposure to the cost of prescription drugs by an average of $2,000 a year.[116] Limits were also placed on out-of-pocket costs for in-network care for Medicare Advantage enrollees. Meanwhile, Medicare Part B and D premiums were restructured in ways that reduced costs for most people while raising contributions from the wealthiest people with Medicare. The law also expanded coverage of preventive services.

The ACA instituted a number of measures to control Medicare fraud and abuse, such as longer oversight periods, provider screenings, stronger standards for certain providers, the creation of databases to share data between federal and state agencies, and stiffer penalties for violators. The law also created mechanisms, such as the Center for Medicare and Medicaid Innovation to fund experiments to identify new payment and delivery models that could conceivably be expanded to reduce the cost of health care while improving quality.