What Is Medicare Part D?
Medicare Part D is the federal program that provides prescription drug coverage to Medicare beneficiaries. It is offered through private insurance companies approved by Medicare and is an optional — but highly recommended — add-on for people enrolled in Original Medicare. Many Medicare Advantage plans also include Part D drug coverage (these are called MAPD plans).
Without Part D (or equivalent employer coverage), you could face a late enrollment penalty if you go without creditable drug coverage for 63 or more consecutive days after becoming eligible.
How Part D Plans Are Structured
Every Part D plan has its own list of covered drugs called a formulary. Drugs are organized into tiers, with lower tiers generally having lower copays.
| Tier | Typical Drug Type | Cost Level |
|---|---|---|
| Tier 1 | Preferred generic drugs | Lowest copay |
| Tier 2 | Non-preferred generics | Low-moderate copay |
| Tier 3 | Preferred brand-name drugs | Moderate copay |
| Tier 4 | Non-preferred brand-name drugs | Higher copay |
| Tier 5 | Specialty drugs | Highest copay/coinsurance |
The Stages of Part D Coverage
Part D coverage works in phases throughout the year. Understanding these stages helps you anticipate costs.
1. Deductible Phase
Many Part D plans have an annual deductible you must pay before the plan begins covering your drugs. (Some plans waive the deductible for lower-tier drugs.) Medicare sets a maximum deductible each year.
2. Initial Coverage Phase
After meeting your deductible, you and your plan share drug costs. You pay a copay or coinsurance for each prescription, and this phase continues until total drug spending (yours plus the plan's) reaches a set threshold.
3. Catastrophic Coverage Phase
Once your out-of-pocket spending reaches the catastrophic threshold for the year, your costs drop significantly. As of 2024 and beyond under the Inflation Reduction Act, your out-of-pocket costs are capped at $2,000 per year (fully in effect in 2025), offering meaningful protection for people on expensive medications.
Choosing the Right Part D Plan
Not all Part D plans are created equal. Here's how to compare them effectively:
- Check the formulary. Make sure all of your current medications are covered at an acceptable tier. Plans change their formularies annually.
- Compare total costs — not just premiums. A plan with a low premium but high copays may cost more than a plan with a slightly higher premium. Use Medicare's Plan Finder tool at medicare.gov to compare total estimated annual costs.
- Verify preferred pharmacies. Many plans have preferred pharmacy networks where your copays are lower. Confirm your preferred pharmacy is included.
- Check star ratings. Medicare rates Part D plans on a 1-to-5 star quality scale. Higher-rated plans tend to provide better service and coverage reliability.
- Consider mail-order savings. Many plans offer lower copays for 90-day supplies through mail-order pharmacies — useful for maintenance medications.
Extra Help: Low-Income Subsidy (LIS)
If your income and resources are limited, you may qualify for the Extra Help program (also called the Low-Income Subsidy). This federal program helps pay for Part D premiums, deductibles, and copays. Eligibility is based on income and asset levels set annually by Medicare. Contact Social Security at 1-800-772-1213 or visit socialsecurity.gov to apply.
Key Reminders
- Formularies change each year — review your plan every fall during Open Enrollment (October 15 – December 7).
- You can switch Part D plans during Annual Enrollment without penalty.
- Always use Medicare's official Plan Finder to compare plans based on your specific medications and pharmacy.
- If you take specialty drugs, pay close attention to tier placement and prior authorization requirements.
Part D can be complex, but taking the time to compare plans each year can save you a significant amount of money and ensure your medications remain covered at an affordable cost.