What happens if you miss the December 7 Medicare Open Enrollment deadline, plus how Part B and Part D late enrollment penalties are calculated.
By the SeniorAffair.com Medicare Team · Reviewed for accuracy against CMS.gov and Medicare.gov
Medicare runs on strict deadlines, and missing one can mean waiting months to fix a coverage gap—or paying a penalty that follows you for as long as you have Medicare. Here’s what actually happens if you miss Open Enrollment (Oct 15–Dec 7, 2026), plus how the Part B and Part D late enrollment penalties are calculated.
Article Contents
The Core Deadline: December 7
Any change you want to make to your Medicare Advantage or Part D coverage for the 2027 plan year must be submitted by December 7, 2026. Submissions are based on the date they’re received, not postmarked, so mailing a paper form on December 6 without accounting for delivery time is a common way people accidentally miss the window.
What Happens If You Miss AEP
| Your Situation | What You Can Still Do |
|---|---|
| Currently in Medicare Advantage | Wait for the Medicare Advantage Open Enrollment Period, Jan 1–Mar 31, for one additional switch or a return to Original Medicare |
| Currently in Original Medicare with a standalone Part D plan | No general fallback window—you’ll likely need to wait until next year’s AEP unless you qualify for a Special Enrollment Period |
| Never enrolled in Part B | Enroll during the General Enrollment Period, Jan 1–Mar 31, with coverage starting the month after you enroll; a late enrollment penalty may apply |
| Experienced a qualifying life event | May be eligible for a Special Enrollment Period regardless of the AEP deadline—see Medicare Special Enrollment Periods: Do You Qualify? |
The Part B Late Enrollment Penalty
If you don’t sign up for Part B when you’re first eligible (and don’t qualify for a Special Enrollment Period, such as still having employer coverage), your monthly Part B premium can go up 10% for each full 12-month period you were eligible but didn’t enroll. Unlike some penalties, this one is permanent—it applies for as long as you have Part B, not just for a year or two.
Example: If you were eligible for Part B for 24 months before enrolling, your premium would be about 20% higher than the standard premium for as long as you’re enrolled in Part B.
The Part D Late Enrollment Penalty
If you go 63 or more consecutive days without Part D or other “creditable” prescription drug coverage (coverage at least as good as standard Part D) after your Initial Enrollment Period ends, you may owe a Part D late enrollment penalty. It’s calculated as roughly 1% of the national base beneficiary premium for each full month you went without creditable coverage, added to your monthly Part D premium—again, permanently, for as long as you have Part D coverage.
Example: Going 24 months without creditable drug coverage would add roughly 24% of the national base beneficiary premium to your monthly Part D premium indefinitely.
If you had prescription drug coverage through an employer, union, or other source that was at least as good as Part D (creditable coverage), you generally won’t owe this penalty as long as you enroll in Part D within 63 days of losing that coverage.
Avoid a penalty by locking in coverage before your window closes. Compare plans available in your area now.
How to Avoid Late Enrollment Penalties
- Enroll in Part B during your Initial Enrollment Period unless you have qualifying employer coverage
- Enroll in a Part D plan (standalone or through Medicare Advantage) as soon as you’re eligible, even if you don’t take many prescriptions currently
- If you have employer or union drug coverage, confirm in writing that it’s considered “creditable” so you can prove it later if needed
- If you lose employer coverage, enroll in Part D within 63 days to avoid a penalty
What If You Made a Mistake During AEP?
If you submitted an enrollment change during AEP and realize afterward it was the wrong choice, you generally cannot reverse it once the window closes on December 7—except through the Medicare Advantage Open Enrollment Period in January–March if you’re in a Medicare Advantage plan, or by qualifying for a Special Enrollment Period. This is one reason it’s worth starting your Open Enrollment review in October rather than making a rushed decision on December 6. For a step-by-step approach to reviewing plans with enough lead time, see Medicare Open Enrollment Checklist: 10 Questions to Ask.
Don’t wait until December 6. See your options now and submit your change with time to spare.
Deadlines That Aren’t AEP
| Deadline | What It’s For |
|---|---|
| 7 months around your 65th birthday | Initial Enrollment Period for Part A and Part B |
| 63 days after losing creditable drug coverage | Enroll in Part D to avoid the late enrollment penalty |
| September 30 | Deadline for plans to mail Annual Notice of Change (ANOC) letters |
| December 7 | Last day to submit an AEP change for a January 1 effective date |
| March 31 | End of both the General Enrollment Period and the Medicare Advantage Open Enrollment Period |
How Late Enrollment Penalties Interact With Star Ratings and Special Enrollment
Some Special Enrollment Periods, such as moving out of your plan’s service area or qualifying for Extra Help, allow you to enroll without triggering a late enrollment penalty even outside AEP. Additionally, if you’re eligible to join a 5-star Medicare Advantage or Part D plan, a special year-round enrollment window applies—see Medicare Advantage Star Ratings Explained and Medicare Special Enrollment Periods: Do You Qualify? for details on both.
If You’re Not Sure Whether You Owe a Penalty
Social Security (for Part B) or your Part D plan (for Part D) will notify you in writing if a late enrollment penalty applies and show you the calculation. If you believe the penalty was calculated incorrectly—for example, if you had creditable employer coverage that wasn’t accounted for — you can request a formal reconsideration and submit proof of your prior coverage.
Why the December 7 Deadline Is Firm
Unlike some administrative deadlines that have informal grace periods, Medicare’s plan enrollment systems are built around fixed processing windows tied directly to the January 1 effective date. Insurers need time between December 7 and January 1 to process enrollment files, issue new member ID cards, and update pharmacy and provider systems. There’s no practical mechanism for a late submission to still take effect January 1, which is why “I meant to do it” isn’t something Medicare can retroactively fix—the systems simply aren’t built to accommodate it.
Common Ways People Accidentally Miss the Deadline
- Mailing a paper enrollment form too close to the deadline. Postal delivery time isn’t factored in—the plan needs to receive it by December 7, not postmark it.
- Assuming a phone call “locked in” a change that was never actually processed. Always ask for a confirmation number and follow up in writing if possible.
- Confusing AEP with the Medicare Advantage Open Enrollment Period. The January–March window is narrower and doesn’t allow switching standalone Part D plans independently.
- Waiting on a decision from a family member who needs to review the options together, and running out the clock in the process.
What to Do If You Realize You’re About to Miss It
If you’re within the final days of AEP and haven’t made a decision, the safest move is to make your best available choice using Medicare.gov’s Plan Finder rather than let the deadline pass entirely. You generally can submit and then still change your mind again before December 7 if a later submission supersedes an earlier one within the same window—confirm this directly with Medicare.gov or 1-800-MEDICARE if you’re unsure, since it’s better to lock in an imperfect improvement than to default into another full year of a plan that no longer fits your needs.
Penalties vs. Coverage Gaps: Two Different Risks
It’s worth separating two distinct risks that people sometimes conflate. A late enrollment penalty is a permanent premium surcharge for delaying enrollment in Part B or Part D beyond your window. A coverage gap is a period where you simply have no active Medicare Advantage or Part D plan at all, which can happen if a plan is discontinued and you don’t select a new one in time. Both are avoidable with the same basic discipline: track your deadlines, respond to plan notices promptly, and don’t assume silence from Medicare means everything is fine by default.
The wider picture ties directly back to the questions covered in the main Open Enrollment guide: know your dates, understand what you’re allowed to change, and build in enough time before December 7 that a missed mail delivery or a busy week doesn’t turn into a full extra year on a plan that no longer serves you well.
Frequently Asked Questions
What happens if I miss the December 7 Medicare Open Enrollment deadline?
If you’re in Medicare Advantage, you can still make one change during the Medicare Advantage Open Enrollment Period from January 1 to March 31. If you’re in Original Medicare with a standalone Part D plan, you generally have to wait until the next Annual Enrollment Period unless you qualify for a Special Enrollment Period.
How much is the Medicare Part B late enrollment penalty?
The Part B penalty adds roughly 10% to your monthly premium for each full 12-month period you were eligible for Part B but didn’t enroll, and it applies permanently for as long as you have Part B.
How is the Part D late enrollment penalty calculated?
It’s approximately 1% of the national base beneficiary premium for each full month you went without creditable prescription drug coverage after your Initial Enrollment Period, added permanently to your monthly Part D premium.
Can I avoid the Part D penalty if I have employer drug coverage?
Yes, as long as your employer or union coverage is considered ‘creditable’ (at least as good as standard Part D), you won’t owe a penalty as long as you enroll in Part D within 63 days of losing that coverage.
Is the Medicare late enrollment penalty permanent?
Yes. Both the Part B and Part D late enrollment penalties are generally permanent additions to your monthly premium for as long as you’re enrolled in that part of Medicare, unless you successfully appeal the penalty.
More in This Medicare Open Enrollment Guide
- Medicare Open Enrollment 2027: The Complete Guide
- Medicare Advantage vs. Original Medicare: How to Decide
- How to Switch Medicare Advantage Plans During Open Enrollment
- Medicare Part D Open Enrollment: Comparing Drug Plans
- Medigap vs. Medicare Advantage: Which Costs Less?
- Medicare Open Enrollment Deadlines and Penalties
- Medicare Advantage Star Ratings Explained
- Medicare Special Enrollment Periods: Do You Qualify?
- Medicare Costs in 2027: Premiums, Deductibles, and Limits
- Medicare Open Enrollment Checklist: 10 Questions to Ask
Not sure your current plan is still your best option?
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