Medicare Part D, also known as the Medicare prescription drug benefit, is a federal program that helps Medicare beneficiaries pay for prescription drugs. It's optional and available to all Medicare recipients. 

Here are some things to know about Medicare Part D:

Coverage: 

• Part D covers brand-name and generic drugs that you take yourself. 

• Plans : Each plan has a list of covered drugs, called a formulary, which can vary in cost and specific drugs covered. 

• Late enrollment penalty: If you join a plan later than you could have, you'll pay an extra 1% for each month you could have signed up for Part D

• The Deductible: Most Part D options have deductibles. However, the
  deductible often does not apply to Tier 1 medications. On some Part D plans, tier 2 drugs are also covered before the deductible. After you reach your deductible, you will often have a copay for your prescriptions. 

• Medications from Tiers 4 and 5: For these more expensive drugs from Tiers 4 and 5, you will often pay a coinsurance percentage. For example, you might pay 30% of the cost
  of the drug. Then, the health insurance carrier would cover the remaining 70%
  of the cost.

• Tier 6 Drugs: Most Part D Plans are on a 5 Tier system. However, some health plans have a sixth tier where they offer certain medications at a reduced cost. Tier 6 prescriptions might include weight loss, hair loss meds, Viagra, or drugs for certain specific medical conditions. If the Part D coverage is designed with certain health conditions in mind, medications related to those conditions might be included in Tier 6. For example, the plan might be focused on those with heart conditions, cancer, or diabetes. And, Tier 6 may include discounted drugs related to those medical conditions. Tier 6 is often not subject to the coverage gap (otherwise known as the “doughnut hole”). 

• No OOP Max on Part D: This is arguably the biggest drawback to Medicare. Medications do not add toward an out-of-pocket maximum. Therefore, there is essentially no limit to how much a Part D member could pay for prescription drugs. For this reason, some seniors at 65 elect to stay on health insurance coverage through their employer. Be aware that staying on employer-sponsored coverage may or may not be an option for you and certain restrictions do apply.