Medicare’s prescription drug benefit (Part D) is the part of Medicare that provides outpatient drug coverage. Part D is provided only through private insurance companies that have contracts with the federal government—it is never provided directly by the government (unlike Original Medicare).
If you want to get Part D coverage, you have to choose and enroll in a private Medicare prescription drug plan (PDP) or a Medicare Advantage Plan with drug coverage (MAPD). Enrollment is optional (though recommended to avoid incurring future penalties) and only allowed during approved enrollment periods. Typically, you should sign up for Part D when you first become eligible to enroll in Medicare.
Whether you should sign up for a Medicare Part D plan depends on your circumstances. You may have creditable drug coverage from employer or retiree insurance. If so, you don’t need to enroll in a PDP until you lose this coverage. Also, some people already enrolled in certain low-income assistance programs may be automatically enrolled in a Medicare drug plan and receive additional financial assistance paying for their medicines.