When Medicare was introduced in 1965, prescription drugs were not covered. However, in December 2003, Medicare Part D was signed into law to cover the cost of medications. Unlike Medicare Part A and Part B, however, Part D is optional.
What Part D Covers
Medicare Part D is offered via plans marketed by private companies. Each plan publishes its own list of covered drugs called a formulary. Formularies contain both brand-name and generic drugs. A formulary typically will have two or more drugs in the most popular categories to allow for more choices.
Formularies are not static. Medicare rules allow a plan to make changes to its formulary and drug fees during the year because: