Original Medicare covers hospital and medical services, but it does not cover most outpatient prescription drugs. That is the gap Medicare Part D fills. Part D is prescription drug coverage offered through private plans approved by Medicare — and understanding how it works can save you real money and real headaches at the pharmacy counter.
Two ways to get Part D coverage
You cannot have two kinds of Part D coverage at once, so you pick one path. If you have Original Medicare (often paired with a Medigap policy), you buy a standalone Prescription Drug Plan that adds drug coverage to your A and B. If you choose a Medicare Advantage plan, drug coverage is usually bundled right in — these are sometimes called MA-PD plans. Either way, the coverage comes from a private plan that follows Medicare’s rules.
Formularies and drug tiers
Every Part D plan publishes a formulary — its list of covered drugs. Drugs on the formulary are grouped into tiers, and your tier determines your share of the cost: lower tiers (often generics) generally cost you less, while higher tiers (often brand-name or specialty drugs) cost more. Two plans can cover the same drug on very different tiers, so the “cheapest” plan on paper is not always the cheapest for your prescriptions. Some drugs also require prior authorization or “step therapy,” meaning you try a lower-cost option first.
How the coverage stages work
Part D coverage moves through stages during the year. You may start by paying a deductible before coverage kicks in, then move into initial coverage where you and the plan share costs. Importantly, there is now a yearly cap on what you pay out of pocket for covered drugs — once you reach it, your covered drug costs for the rest of the year are minimal. That cap is one of the most meaningful protections Part D offers, especially for people who take expensive medications.
Why you should review your plan every fall
Part D plans can change every year: premiums, deductibles, which drugs are covered, which tier your drugs sit on, and which pharmacies count as “preferred.” A plan that was perfect for you last year can quietly become expensive this year if one of your drugs moved to a higher tier. The Annual Enrollment Period — October 15 through December 7 — is your yearly window to compare plans and switch. It is worth an hour of your time.
Practical tips for choosing a Part D plan
- Make a list of every prescription you take, including names and dosages, before you compare plans.
- Check each plan’s formulary for your drugs — not just the plan’s overall rating.
- Look at the tier placement of your drugs, not just the monthly premium.
- Confirm your pharmacy is in the plan’s network — “preferred” pharmacies often mean lower costs.
- Ask your pharmacist whether a generic or therapeutic alternative exists for expensive brand drugs.
- If you delay enrolling without other creditable drug coverage, a late-enrollment penalty may apply later.
This article is general information, not personal advice. For official details, visit medicare.gov or call 1-800-MEDICARE.