Every fall, a window opens.

From October 15th through December 7th, Medicare beneficiaries have the opportunity to review their current coverage, compare their options, and make changes that take effect January 1st. It is one of the most important financial decisions many retirees make each year.

It is also one of the most commonly mishandled.

Not because people are careless. But because Medicare is genuinely complicated, the stakes are not always obvious until something goes wrong, and the deadlines are unforgiving in ways that catch people off guard.

Here are the mistakes that come up most often and what to do about them before December 7th.

Mistake 1: Doing Nothing Because Last Year’s Plan Felt Fine

This is by far the most common mistake, and it is an easy one to make.

Your plan worked well enough last year. You did not have major issues. So you let the enrollment period pass without reviewing anything, and your coverage rolls over automatically.

The problem is that Medicare plans change every year. Premiums shift. Drug formularies get updated, meaning medications that were covered last year may be covered differently or not at all this year. Provider networks can change. A plan that was the right fit in 2025 may be meaningfully more expensive or less comprehensive in 2026 with no notice beyond the Annual Notice of Change letter that most people do not read carefully.

Taking 30 minutes to review your current plan against your alternatives each year is one of the simplest and highest-value things a Medicare beneficiary can do. The cost of not doing it can show up quietly in the form of higher premiums, larger out-of-pocket costs, or a prescription that is suddenly much more expensive than it used to be.

Mistake 2: Not Reviewing Part D Against Your Actual Prescriptions

Medicare Part D prescription drug plans vary significantly from one another, and the right plan depends almost entirely on the specific medications you take.

The most common mistake here is choosing a plan based on the monthly premium alone. A lower premium plan may look attractive on the surface, but if your medications are not on that plan’s formulary or are placed in a higher cost-sharing tier, you can end up paying significantly more out of pocket than you would have with a slightly higher premium plan that covers your drugs more favorably.

Medicare’s website includes a comparison tool that allows you to enter your specific medications and compare what each plan would actually cost you over the course of the year, including both premiums and drug costs. Taking the time to use it is the only way to make a genuinely informed Part D decision, and it is worth the effort before the December 7th deadline.

Mistake 3: Confusing the Annual Enrollment Period With Other Enrollment Windows

Medicare has multiple enrollment periods, and they are not interchangeable.

The Annual Enrollment Period running now, October 15th through December 7th, is for switching between Medicare Advantage plans, switching from Medicare Advantage back to Original Medicare, and changing Part D prescription drug plans.

It is not the window for adding a Medicare Supplement plan without medical underwriting. It is not the same as the Medicare Advantage Open Enrollment Period that runs January 1st through March 31st. And it is not a general opportunity to make any Medicare change you want.

Understanding which window does what matters because acting during the wrong period, or missing the right one, can mean waiting another year or facing medical underwriting requirements that may not work in your favor.

Mistake 4: Choosing a Plan Based on Premium Alone

A zero-dollar premium Medicare Advantage plan is genuinely appealing. And for some people it is the right choice.

But the premium is only one piece of the total cost picture. Deductibles, copayments, coinsurance, and out-of-pocket maximums all vary significantly between plans, and a plan with a low premium can expose you to substantially higher costs if you have significant healthcare needs during the year.

Evaluating a plan based on its true total cost, not just the monthly premium, gives you a much more accurate picture of what you are actually signing up for.

Mistake 5: Not Checking Whether Your Doctors Are Still in Network

Provider networks change. A physician, specialist, or hospital that was in your Medicare Advantage plan’s network last year may not be in it next year.

If you have preferred providers or ongoing care relationships that matter to you, confirming that they are still in network before your plan renews is worth doing every year. Discovering mid-January that your doctor is no longer covered is a much more stressful situation than taking ten minutes to verify before December 7th.

Mistake 6: Ignoring IRMAA When Making Income Decisions

This one sits at the intersection of Medicare and broader financial planning, which is exactly why it belongs in this conversation.

Medicare Part B and Part D premiums are based on your income from two years prior. If your income exceeds certain thresholds, you pay more through a surcharge called IRMAA. What many people do not realize is that a single large taxable event, an IRA withdrawal, a Roth conversion, or the sale of an asset, can push you into a higher IRMAA tier and increase your Medicare premiums for an entire year.

This is one of the clearest examples of why Medicare decisions and financial planning need to happen in the same conversation. A decision that looks purely financial can have a direct and meaningful impact on your healthcare costs.

The Window Closes December 7th

Changes made during the Annual Enrollment Period take effect January 1st. That gives you a few weeks to review your options, compare plans, and make any changes before the deadline.

If you have not looked at your Medicare coverage recently, this is the right time. And if you are not sure how your Medicare choices interact with your broader retirement financial picture, that is exactly the kind of conversation we have every day.

Want to make sure your Medicare coverage is working as hard as it should be? Schedule a complimentary first visit with the team at True Financial Partners and let’s take a look at the full picture together.

Summary:

Every year, Medicare’s Annual Enrollment Period opens a critical window to review and change your coverage. And every year, people make the same costly mistakes during it. This post breaks down the most common Medicare enrollment errors, why they happen, and what to do instead before the December 7th deadline closes the window for another year.

Frequently Asked Questions

When is Medicare’s Annual Enrollment Period?
It runs October 15th through December 7th each year. Changes made during this window take effect January 1st of the following year.

Can I be penalized for missing the Medicare enrollment window?
Missing your initial enrollment window can result in permanent late enrollment penalties on Part B and Part D premiums. Missing the Annual Enrollment Period means your current coverage rolls over and you wait until the next year to make changes.

What can I change during the Annual Enrollment Period?
You can switch between Medicare Advantage plans, move from Medicare Advantage back to Original Medicare, or change your Part D prescription drug plan. It is not the window for adding a Medicare Supplement plan without medical underwriting.

Why does my Medicare plan need to be reviewed every year?
Because plans change annually. Premiums, drug formularies, and provider networks can all shift from one year to the next, meaning a plan that was the right fit last year may not be the best option this year.

What is IRMAA and why does it matter during enrollment?
IRMAA is a surcharge added to Medicare Part B and Part D premiums for higher-income beneficiaries. Because it is based on income from two years prior, large taxable events can unexpectedly increase your Medicare costs. It is one reason Medicare planning and financial planning should always happen together.