Medicare Part D prescription drug plans, explained
Original Medicare doesn't cover most prescriptions — that's Part D's job. Here's how to pick the plan that actually covers your meds.
Original Medicare (Parts A and B) doesn't cover most prescription drugs, so Part D is how you get drug coverage. With dozens of plans available in most areas, the trick is choosing based on your specific medications — not the lowest premium.
What Part D covers
Part D plans are sold by private insurers and cover prescription drugs. Each plan has a 'formulary' — its list of covered drugs, organized into tiers that determine your copay. Many Medicare Advantage plans bundle Part D, or you can buy a standalone Part D plan alongside Original Medicare.
Why the formulary matters most
The single biggest factor is whether your drugs are on the plan's formulary, and on what tier. A plan with a cheap premium can be expensive if your medications sit on a high tier — or aren't covered at all. Always check your exact prescriptions against the formulary before enrolling.
How to choose
- List every medication and dosage you take.
- Check each plan's formulary and tier for those drugs.
- Compare total annual cost — premium + deductible + copays — not just the premium.
- Confirm your preferred pharmacy is in-network.
When you can enroll or switch
You can join during your Initial Enrollment Period around 65, and change plans each year during the Annual Enrollment Period (October 15–December 7). Going without creditable drug coverage can trigger a permanent late-enrollment penalty, so don't skip it.
Frequently asked questions
Often yes — enrolling when first eligible avoids a permanent late-enrollment penalty later, and protects you if you need medications in the future. A low-cost plan can serve as inexpensive insurance against that penalty.
Match the plan to your medications: check that each drug is on the formulary and what tier it's on, then compare the total annual cost (premium + deductible + copays), not just the monthly premium.
During the Annual Enrollment Period, October 15–December 7, with changes effective January 1.
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