Understanding Medicare Part D prescription drug plans

Part D is the piece of Medicare that covers prescription drugs. It doesn't come automatically with Original Medicare, and not every Medicare Advantage plan includes it either. Here is who typically needs a stand-alone Part D plan, how these plans are generally structured, and a rule worth understanding before you decide to wait.

Who typically needs a stand-alone plan

If you have Original Medicare — Part A and Part B — and nothing else, you have no prescription drug coverage unless you add a stand-alone Part D plan. The same is true if you're in a Medicare Advantage plan that doesn't bundle drug coverage in; you would need a separate Part D plan alongside it, since most Advantage plans that do include drug coverage don't allow you to also carry a stand-alone Part D plan on top. If you already have creditable prescription drug coverage from another source — for example, through an employer or union plan — you may not need a separate Part D plan right away, but it's worth confirming that coverage is considered creditable before you decide.

How formularies and tiers generally work

Every Part D plan has a formulary — a list of the drugs it covers. Formularies are typically organized into tiers, with lower tiers generally covering generic drugs at a lower cost to you and higher tiers generally covering brand-name or specialty drugs at a higher cost. Which drugs sit on which tier, and how much you pay at each tier, varies from plan to plan and can change from year to year. Two Part D plans covering the same drug can charge different amounts for it, and a plan that fit your prescriptions well last year isn't guaranteed to fit them the same way next year — which is part of why it's worth reviewing your plan against your actual medication list on a regular basis, not just the year you first enrolled.

The late enrollment penalty

This is a real rule set by CMS, and it's worth taking seriously even if you don't take many prescriptions today. If you go without Part D coverage — or other drug coverage considered creditable, meaning it's expected to pay at least as much as a standard Part D plan — for an extended period after you're first eligible for Medicare, a permanent penalty can be added to your Part D premium if you enroll later. That penalty generally stays with you for as long as you have Part D coverage. The exact calculation depends on how long you went without creditable coverage, and CMS is the authoritative source for how it's figured — the point to understand here is simply that “I'll add drug coverage later if I need it” can end up costing more than signing up when you're first eligible, even if you're not filling many prescriptions right now.

Choosing a plan isn't one-size-fits-all

Because formularies, tiers, and costs vary by plan and by drug, the right Part D plan for one person can be a poor fit for another, even if they live in the same building. The most useful comparison starts with your actual medication list, not a plan's advertised premium. A plan with a low premium but a formulary that doesn't cover your prescriptions well can end up costing more overall than a plan with a higher premium and better coverage for what you actually take.