Medicare part b special enrollment period rules are what let you delay Part B while you are still working, without paying a lifetime penalty for the delay. If you have group health plan coverage based on your own or your spouse’s current employment, you generally do not have to take Part B at 65. When that job or that coverage ends, a clock starts. You get 8 months to sign up for Part B under the Special Enrollment Period.
Miss the window and you are pushed into the General Enrollment Period, with a penalty that follows you for life. The medicare part b special enrollment period is described on the official Medicare sign-up page. This guide walks the process, the forms and the traps.
The Rule Behind Medicare Part B Special Enrollment Period
Everything turns on employer size and on the words “current employment.” Under the Medicare Secondary Payer rules, a group health plan at an employer with 20 or more employees pays primary for workers 65 and over when coverage is based on current employment. Medicare pays second. That is the situation the medicare part b special enrollment period was written for.
At an employer with fewer than 20 employees, the order flips. Medicare generally pays first and the group plan pays second. CMS explains the payment order on the Who pays first page.
That difference matters more than the penalty math. If you work for a small employer and skip Part B, your group plan may pay only the share it would have paid as secondary. You could be left holding the bill Medicare would have covered. Confirm the primary payer question with your plan administrator in writing, not with a coworker.
Multi-employer and multiple employer plans can follow different size counting rules. The medicare part b special enrollment period still exists in those cases, but who pays first may not match the simple 20-employee rule. Ask your benefits office how your plan counts employees.
Your Choices at 65
You have three practical paths. Take Part A only and keep working. Take Part A and Part B together. Or delay both and enroll later. Most people with large-employer coverage take premium-free Part A and delay Part B, then use the medicare part b special enrollment period when work ends. That is a choice, not a requirement.
Part A is premium-free if you or your spouse have at least 40 quarters of Medicare-covered employment. If you pay a premium for Part A, delaying it may make sense. If you contribute to an HSA, taking Part A has tax consequences covered further down.
| Your situation | Who pays first | What to consider doing |
|---|---|---|
| Employer with 20+ employees, you or your spouse still actively working | Group health plan pays first, Medicare second | You may delay Part B and use the 8-month Special Enrollment Period after work or coverage ends |
| Employer with fewer than 20 employees, still actively working | Medicare pays first, group plan second | Enrolling in Part B at 65 is usually discussed; confirm with the plan administrator before declining |
| COBRA after employment ends | Medicare pays first | COBRA is not current employment coverage; the 8-month clock runs from when the job or job-based coverage ended |
| Retiree health coverage | Medicare pays first | Retiree coverage does not extend the Special Enrollment Period; enroll in Part B on schedule |
| Coverage through a spouse’s current job at a large employer | Spouse’s group plan pays first | The Special Enrollment Period applies when the spouse’s employment or coverage ends |
| Marketplace, VA-only, or short-term coverage | Medicare pays first | These are not group health plans based on current employment and do not qualify you for the SEP |
Read the table as a starting point for a conversation with Social Security and your benefits office. It does not replace either one.
The Penalty If You Get It Wrong
The Part B late enrollment penalty is 10% of the standard premium for each full 12-month period you could have had Part B but did not. Two full years late is a 20% penalty. Medicare states on its penalties page that you usually pay it for as long as you have Part B.
The arithmetic uses the standard premium, which CMS set at $202.90 per month for 2026 in its 2026 premiums fact sheet. A 20% penalty on that base is about $40.58 a month on top of the premium. The base premium changes each year, so the dollar penalty moves with it.
Part D has its own penalty. It is 1% of the national base beneficiary premium, times the number of full uncovered months without creditable drug coverage. Medicare lists that base as $38.99 for 2026 on its Part D penalty page.
Twenty-four uncovered months is 24%, roughly $9.36 a month, rounded to the nearest 10 cents. That amount is added to your drug plan premium for as long as you have Part D coverage, even if you switch plans. Using the medicare part b special enrollment period correctly avoids the Part B penalty. It does not by itself protect you from the Part D penalty.
Medicare Part B Special Enrollment Period: the Paperwork
Two forms do the work. Form CMS-40B, Application for Enrollment in Medicare Part B, is the application itself. You complete it and you sign it. Nobody else signs the CMS-40B.
Form CMS-L564, Request for Employment Information, is the proof. You complete Section A yourself. Your employer or the plan administrator completes and signs Section B, confirming the dates of group health plan coverage based on current employment. Without a signed Section B, a medicare part b special enrollment period request is usually incomplete.
If your employer will not sign, Social Security accepts secondary evidence in some cases, such as pay stubs, W-2 forms or health insurance cards showing the coverage dates. Ask the Social Security office what it will accept before you send anything.
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Submit both forms to your local Social Security office by mail or fax, or apply online through SSA’s Part B sign-up page. Social Security also publishes a short guide to applying during a Special Enrollment Period.
Timing is the part people underestimate. The medicare part b special enrollment period runs 8 months from the month after employment ends, or the month after the group coverage ends, whichever comes first. Coverage start dates depend on when you file. Ask Social Security for your exact effective date in writing, and keep a copy of everything you send.
The Traps People Fall Into
COBRA is the most common one. COBRA is not coverage based on current employment. Medicare’s COBRA page is direct about this. Taking 18 months of COBRA does not add 18 months to the medicare part b special enrollment period. The clock already started when the job ended.
Retiree coverage works the same way. It is not current employment coverage, and it does not preserve the medicare part b special enrollment period. Neither does a Marketplace plan.
HSA contributions are the tax trap. IRS Publication 969 states that beginning with the first month you are enrolled in Medicare, your HSA contribution limit is zero. Part A can be retroactive up to 6 months when you enroll after 65. Contributions made during a retroactive period become excess contributions. Talk to a tax professional before your last HSA deposit.
Small employers are the quiet trap. If your employer has fewer than 20 employees, delaying Part B can leave gaps your group plan will not fill. Ask the plan administrator directly whether the plan pays primary or secondary for workers over 65.
Frequently Asked Questions
When exactly does the 8-month clock start?
It starts the month after your employment ends or the month after your group health plan coverage based on current employment ends, whichever happens first. If those dates differ, confirm which one Social Security is using for your record.
Do I need Part A before I can use the medicare part b special enrollment period?
No. You can enroll in Part B alone using CMS-40B and CMS-L564 if you already have Part A, or apply for both. SSA’s Part B sign-up page explains the paths and the online option.
My employer’s HR will not complete CMS-L564. What now?
Ask Social Security about secondary evidence of coverage, such as pay stubs, W-2 forms or insurance cards showing dates. Do not assume a substitute is acceptable until Social Security tells you so.
Does the medicare part b special enrollment period cover Part D too?
Part D has a separate Special Enrollment Period, generally 2 months after creditable drug coverage ends. That is a shorter window than the medicare part b special enrollment period. Confirm your drug coverage end date and your Part D deadline with Medicare or your plan.
Compare Your Options
Once you know what medicare part b special enrollment period means for you, the official Plan Finder shows what is actually sold in your ZIP code, and a SHIP counselor will walk it with you for free — no commission, no sales call.
Official Sources & Resources
- Social Security — Medicare enrollment: https://www.ssa.gov/medicare
- IRS Publication 969 (HSAs): https://www.irs.gov/publications/p969
- Medicare.gov: https://www.medicare.gov
- CMS.gov: https://www.cms.gov
- Find your SHIP counselor: https://www.medicare.gov/talk-to-someone
Checked against the official sources above in September 2026. Rules and dollar figures change; if a notice you received disagrees with this page, the notice wins — and please tell us. General information, not legal, financial or medical advice.