Medicare Part D, the prescription drug benefit, is the part of Medicare that covers most outpatient prescription drugs. Part D is offered by medicare providers in Pennsylvania either as a stand-alone plan, for those enrolled in Original Medicare, or as a set of benefits included with your Medicare Advantage Plan. Part D plan has a list of covered drugs, called its formulary. If your drug is not on the formulary. A drug category is a group of drugs that treat the same symptoms or have similar effects on the body.