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Medicare Prescription Drug Coverage (Part D): What It Covers, What It Costs, and How to Choose the Right Plan

If you take even one regular medication, Medicare prescription drug coverage can make a meaningful difference in your monthly budget. But Part D can also feel confusing because plans change, formularies change, and costs can vary widely from one person to the next—even within the same ZIP code.

This guide explains how Medicare prescription drug coverage works, what it typically covers, and the key steps to choosing a plan that fits your medications and your budget.

What Is Medicare Part D?

Medicare Part D is the part of Medicare that helps cover outpatient prescription drugs (the medications you pick up at a pharmacy or receive by mail order).

You can get drug coverage in one of two main ways:

  1. Standalone Part D Plan (PDP):
    Added to Original Medicare (Part A and Part B).

  2. Medicare Advantage Plan with Drug Coverage (MAPD):
    A Medicare Advantage plan (Part C) that includes prescription coverage.

If you want prescription drug coverage, you generally need to enroll in one of these options—Original Medicare by itself does not include routine outpatient drug coverage.

What Part D Typically Covers (and What It Doesn’t)

Commonly covered:

Most Part D plans cover a broad range of medications, including:

  • Brand-name and generic drugs

  • Common maintenance medications (blood pressure, cholesterol, diabetes, etc.)

  • Many specialty medications (depending on the plan)

  • Vaccines (many adult vaccines are covered under Part D)

Not typically covered:

Part D plans usually do not cover:

  • Drugs administered in a hospital or skilled nursing setting (often Part A)

  • Many drugs administered in a doctor’s office (often Part B)

  • Over-the-counter medications

  • Certain categories that may be excluded under federal rules (varies by plan and situation)

How Part D Plans Decide What You Pay

Part D costs are not “one-size-fits-all.” Your total cost depends on your plan, your medications, your pharmacy, and your usage.

Here are the biggest factors:

1) Monthly premium

Some plans have low premiums, but that doesn’t automatically mean lower total cost. A higher premium plan can sometimes reduce your copays enough to save money overall—especially for people with multiple prescriptions.

2) Deductible

Many plans have a deductible (some don’t). If your plan has one, you may pay more upfront until the deductible is met.

3) Copays and coinsurance

Once coverage starts, you’ll typically pay either:

  • a copay (fixed dollar amount), or

  • coinsurance (a percentage of the drug cost)

4) The plan’s formulary (drug list)

Each plan has a formulary—a list of covered drugs. Drugs are usually placed into tiers, and tiers influence what you pay:

  • Lower tiers (preferred generics) usually cost less

  • Higher tiers (non-preferred brands or specialty meds) cost more

5) Pharmacy network: preferred vs. standard

Many plans offer better pricing at preferred network pharmacies. Using a non-preferred pharmacy can significantly increase costs, even if the plan still “covers” the medication.

6) Prior authorization, step therapy, and quantity limits

Some medications may require extra steps:

  • Prior authorization: the plan must approve the drug before it’s covered

  • Step therapy: you may need to try a lower-cost alternative first

  • Quantity limits: limits on how much you can get at one time

The Most Common Mistake: Choosing by Premium Alone

It’s tempting to pick the lowest premium plan—but that can backfire if:

  • your drugs aren’t covered (or are covered on a high tier),

  • your pharmacy isn’t preferred, or

  • your copays are much higher than a slightly higher premium plan.

The best plan is usually the one with the lowest estimated total annual cost for your specific medication list.

How to Choose the Right Part D Plan

Here’s a simple checklist that works well for most beneficiaries:

Step 1: Make a complete medication list

Include:

  • drug name

  • dosage

  • frequency

  • whether you use name brand or generic

  • the pharmacy you prefer (or if you’re open to switching)

Step 2: Confirm each medication is covered

Look for:

  • whether it’s on the plan’s formulary

  • what tier it’s on

  • any restrictions (prior authorization, step therapy, quantity limits)

Step 3: Compare costs at your pharmacy

Check pricing at:

  • preferred network pharmacies

  • mail order (if you’re open to it)

Step 4: Consider flexibility and predictability

If you prefer stable budgeting, a plan with slightly higher premium but lower copays may feel easier month-to-month.

Step 5: Re-check every year

Plans can change their formularies, tiers, and pharmacy networks. Even if your plan worked great last year, it’s smart to review it annually.

When You Can Enroll or Change Part D

You can typically enroll or change plans:

  • When you first become eligible for Medicare

  • During the annual enrollment period (commonly in the fall)

  • During certain special enrollment periods (SEPs), depending on circumstances

  • If you qualify for Extra Help or certain other programs (rules vary)

If you go without creditable drug coverage for too long, you may face a late enrollment penalty, so timing matters.

Extra Help: A Program Many People Miss

If you have limited income and resources, you may qualify for Extra Help (Low-Income Subsidy), which can reduce:

  • premiums

  • deductibles

  • copays

Many people who qualify don’t realize it, so it’s worth checking.

Quick Takeaways

  • Part D helps cover outpatient prescription drugs through standalone plans or Medicare Advantage plans with drug coverage.

  • Your total cost depends on your medications, your pharmacy, and the plan’s formulary and tiers—not just the premium.

  • The “best” plan is usually the one with the lowest total estimated annual cost for your drug list.

  • Reviewing your coverage yearly can prevent surprise cost increases.

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