The short answer: Part D is Medicare’s prescription drug coverage, sold by private insurance companies either as a standalone plan alongside Original Medicare or built into most Medicare Advantage plans. Every plan has its own formulary, the list of drugs it covers and what tier each one sits on, which is why two plans with similar premiums can differ by thousands of dollars a year for the same person.

Pick it based on your actual prescriptions, not the monthly premium.

How does Part D actually work?

You get drug coverage one of two ways:

  • A standalone Part D plan that sits alongside Original Medicare, usually paired with a Medicare Supplement, which never includes drug coverage.
  • A Medicare Advantage plan that includes drug coverage, which is how most Advantage plans are sold.

You generally cannot stack both. If you are weighing those two structures in the first place, start with Medicare Advantage vs Medicare Supplement.

What is a formulary, and why does it decide everything?

The formulary is the plan’s list of covered drugs, sorted into tiers. Lower tiers are generic and cheap, higher tiers are brand name and specialty, and your cost depends on which tier your drug lands on with that specific plan.

Two other things live in the formulary and cost people real money:

  • Prior authorization. The plan requires approval before it will cover a drug.
  • Step therapy. The plan requires you to try a cheaper drug first.

Neither is unusual and neither is a reason to avoid a plan by itself. They are a reason to check your specific medications against a specific plan before enrolling, rather than after your pharmacist tells you at the counter.

This is the whole reason a low premium can be a bad deal. A plan that costs a few dollars a month and puts one of your maintenance drugs on a high tier can cost you far more over a year than a plan with a higher premium that covers it well.

Is there a cap on what I pay?

Yes, and this is the biggest change to Part D in years.

Part D now has an annual cap on your out-of-pocket costs for covered drugs. Once you hit it, covered drugs cost you nothing for the rest of the calendar year. The dollar figure is adjusted annually, so the durable thing to know is that the ceiling exists.

For years there was no ceiling at all. Anyone on an expensive specialty drug simply kept paying a share indefinitely. If you or a family member stopped filling a prescription in the fall because of cost, that math has changed and it is worth revisiting.

There is also now an option to spread your out-of-pocket drug costs across the calendar year in monthly amounts rather than paying them at the pharmacy counter as they hit. It does not reduce what you owe. It changes when you owe it, which helps most if you face a large bill in January and helps very little if your costs are low and steady.

What is the late enrollment penalty?

If you go without creditable drug coverage after you are first eligible, a penalty of roughly 1 percent of a national base premium for each full month you waited gets added to your Part D premium. For most people it never comes off.

Creditable coverage means drug coverage at least as good as Part D. Employer plans, the VA, and TRICARE commonly qualify. Keep the notices those plans send you, because they are the proof.

The practical trap: someone with no prescriptions decides Part D is a waste of money at 65, stays off it for six years, then gets a diagnosis. The drug is covered, but the penalty rides along with the premium from then on. Many people in that situation take an inexpensive plan at 65 purely to stop the clock. The related timing rules are in when to enroll in Medicare.

When can I change drug plans?

Mainly during the Annual Enrollment Period, October 15 through December 7, with the new plan starting January 1. That is the window where you can join, switch, or drop a drug plan for any reason.

Change it every year, or at least check it every year. Formularies shift each January and so do tiers and pharmacy networks, which means a plan that fit you perfectly can quietly stop fitting without anything about your health changing. Our Annual Enrollment Period guide covers the full window.

If money is tight

There is a federal program, usually called Extra Help, that lowers Part D costs for people with limited income and resources. It can reduce or eliminate the premium and deductible and cut what you pay per prescription, and it also protects you from the late enrollment penalty.

Eligibility is based on income and resources, and the thresholds move, so the honest answer is that it is worth checking rather than assuming you earn too much. It is one of the most under-claimed benefits in Medicare, and we ask about it as a matter of course at our office in Sumter.

How to compare plans properly

Bring three things and the comparison takes about fifteen minutes:

  • Your full prescription list, with doses. Doses matter, because tiers can differ by strength.
  • Your pharmacy. Plans have preferred pharmacies, and the same drug can cost different amounts at different counters on the same plan.
  • Last year’s actual spend, if you have it.

Then compare on estimated annual total cost, not premium. Premium is one line in the math. A local Medicare agent can run your drug list across the plans available in your county at no cost to you.

Frequently asked questions

Do I have to take Part D if I do not take any prescriptions? You are not required to, but going without it has a cost. If you do not have other creditable drug coverage and you sign up later, a late enrollment penalty is added to your premium for as long as you have Part D. Many people with no current prescriptions take a low cost plan purely to keep the penalty clock from starting.

What is the Part D late enrollment penalty? Roughly 1 percent of a national base premium for every full month you went without creditable drug coverage after you were first eligible. It is added to your monthly premium and, for most people, it is permanent. Coverage from an employer, the VA, or TRICARE usually counts as creditable, which is why keeping those notices matters.

Is there a limit on what I pay for drugs each year? Yes. Part D now has an annual cap on your out-of-pocket costs for covered drugs, and once you reach it, covered drugs cost you nothing for the rest of the calendar year. The dollar amount is adjusted each year. Before this cap existed there was no ceiling at all, which is the single biggest improvement to Part D in years.

Why is my drug not covered when it was covered last year? Because formularies change every January. A plan can move a drug to a higher tier, add prior authorization or step therapy, or drop it entirely. This is why the plan that was right for you last year is not automatically right this year, and why the Annual Enrollment Period matters even when nothing about your health has changed.

Can I spread my drug costs out over the year? There is now an option to pay your out-of-pocket drug costs in monthly amounts across the calendar year instead of all at once at the pharmacy counter. It does not lower what you owe in total, it changes the timing. It tends to help people facing a large bill early in the year and does little for those with steady low costs.

Griffin Insurance Agency is not connected with or endorsed by the United States government or the federal Medicare program. We do not offer every plan available in your area. Currently we represent 7 organizations which offer 48 products in your area. Please contact Medicare.gov or 1-800-MEDICARE to get information on all of your options.