Medicare Part D, the prescription drug benefit, is the part of Medicare that covers most outpatient prescription drugs. Part D is offered through private companies either as a stand-alone plan, for those enrolled in Original Medicare, or as a set of benefits included with your Medicare Advantage Plan.

Unless you have creditable drug coverage and will have a Special Enrollment Period, you should enroll in Part D when you first get Medicare. If you delay enrollment, you may face gaps in coverage and enrollment penalties.

Each Part D plan has a list of covered drugs, called its formulary. If your drug is not on the formulary, you may have to request an exception, pay out of pocket, or file an appeal.

A drug category is a group of drugs that treat the same symptoms or have similar effects on the body. All Part D plans must include at least two drugs from most categories and must cover all drugs available in the following categories:

• HIV/AIDS treatments
• Antidepressants
• Antipsychotic medications
• Anticonvulsive treatments for seizure disorders
• Immunosuppressant drugs
• Anticancer drugs (unless covered by Part B)

Part D plans must also cover most vaccines, except for vaccines covered by Part B.

Some drugs are explicitly excluded from Medicare coverage by law, including drugs used to treat weight loss or gain, and over-the-counter drugs.

Note: For certain drugs or under specific circumstances, your drugs may be covered by Part A or Part B.