What drugs are covered by your Part D depends on the type of drug. Every Medicare drug plan provides a list of drugs it covers. This list is called a formulary, and the drugs are organized in tiers depending on what they are. You can generally expect to pay more for a drug listed in the higher tier than a drug listed in the lower tier, though you or your prescriber can ask your plan for an exception in some instances.
Prescription drugs can cost a fortune, so it is important to have a coverage plan no matter what Medicare service you are using. Enrolling in a Part D plan can affect whatever coverage you already have, so make sure you speak with an experienced agent to determine which route saves you the most money and gets you the best level of care.