Medicare Part D Explained: How Prescription Drug Coverage Works

Medicare Part D is the prescription drug benefit that helps cover the cost of medications. Whether you have Original Medicare or a Medicare Advantage plan, understanding how Part D works can save you hundreds — even thousands — of dollars each year. This guide explains the coverage phases, formulary tiers, and strategies for minimizing your drug costs.

How Part D Coverage Works: The Four Phases

Part D coverage moves through four distinct phases during the calendar year. The amount you pay changes as you move through each phase.

Understanding Formulary Tiers

Every Part D plan has a formulary — a list of covered medications organized into tiers. The tier determines how much you pay for each drug.

Formularies change every year, and each plan's formulary is different. A drug that's Tier 1 on one plan might be Tier 3 on another — or not covered at all. This is why it's critical to check your specific medications against a plan's formulary before enrolling.

How to Get Part D Coverage

There are two ways to get Part D prescription drug coverage:

• Standalone Part D Plan (PDP): Purchased separately to supplement Original Medicare. You'll have a separate card and premium for drug coverage.
• Medicare Advantage with Drug Coverage (MA-PD): Many Medicare Advantage plans include Part D drug coverage as part of the plan. This is the more convenient option if you're already in an MA plan.

Choosing the Right Part D Plan

1. Make a list of all your current prescriptions, including dosages
2. Use Medicare.gov's Plan Finder to enter your medications and compare plans
3. Look at total annual cost — not just the monthly premium
4. Check if your pharmacy is in the plan's preferred pharmacy network
5. Verify that all your medications are on the plan's formulary
6. Consider mail-order pharmacy options for additional savings
7. Review any coverage restrictions (prior authorization, step therapy, quantity limits)