When you have Medicare plus another plan, getting the order of payment wrong costs real money, and one common COBRA mistake triggers a penalty that lasts for life. Medicare coordination of benefits is the set of rules that decides which of your plans pays first, whether that's your employer, a retiree plan, COBRA, or Medicare itself.

How Medicare coordination of benefits works: primary vs. secondary payers

When you have Medicare and other coverage at once, one plan is the "primary payer" and the other the "secondary payer." The primary payer processes your claim first, up to the limits of its coverage; whatever remains, including deductibles, coinsurance, and anything it didn't cover, then goes to the secondary payer.

Which plan gets which role is the hard part: the same two plans swap places depending on your age, whether you're still working, how many employees your employer has, and whether your Medicare came from age or a medical condition.

If you're unsure which plan pays first in your own situation, contact your plan or call the Benefits Coordination & Recovery Center at 1-855-798-2627 (TTY 1-855-797-2627).

Employer coverage: the employee count changes everything

Two questions decide this, not one: what kind of plan you have, and how many employees your employer has. And the count is not the same for everyone: 20 if your Medicare comes from being 65 or older, 100 if it comes from disability before 65.

Large employer plans (20 or more employees): If you're 65 or older and actively working, your employer has 20 or more employees, and your group health plan is not a tribal health plan, your employer group health plan pays primary. Medicare pays secondary. The same order applies if your employer is part of a multi-employer group health plan in which at least one company has 20 or more employees. This means your employer plan processes claims first. If you skip Medicare enrollment during this period, that's generally allowed without penalty, because you have qualifying current employer coverage. One warning travels with this branch: if the plan paying first is an HMO or an employer PPO and you get services outside its network, it's possible that neither the plan nor Medicare will pay, so call the plan before you go out of network.

Small employer plans (fewer than 20 employees): If your employer has fewer than 20 employees, Medicare becomes the primary payer, and the employer plan pays secondary. Enroll when you're first eligible: if Medicare is the primary payer and you haven't enrolled, your employer plan may deny coverage for the portion it would have paid as secondary. Medicare's who-pays-first rules confirm the threshold.

Tribal health plans: If your health insurance is through a tribal health plan, the order runs the other way from the 20-or-more-employee rule above. Medicare pays first and the tribal health plan pays second, regardless of employer size. If you get drugs through an Indian health facility, you'll continue to get them at no cost to you, and your coverage won't be interrupted.

If your Medicare comes from disability rather than age, the threshold is 100 employees, not 20. If you're under 65, entitled to Medicare because of disability, and covered through your own or a family member's current employment, the plan counts as a "large group health plan" and pays primary, with Medicare secondary, when at least one employer in the plan employed 100 or more full-time and/or part-time employees on 50 percent or more of its business days in the previous calendar year. Below 100 employees Medicare generally pays first, unless the plan is a multi-employer plan in which at least one participating employer has 100 or more employees, in which case Medicare stays secondary. Put concretely: at a 50-employee company, your employer plan pays first if your Medicare came from turning 65, and Medicare pays first if it came from disability.

All of that applies to active employment. Once employment ends, the rules shift.

Per the Medicare.gov working-past-65 guide, you can delay Part B without penalty while you have coverage through a current employer of 20 or more employees. Not every job-based plan counts: Medicare gives this guidance for group health insurance that is available to everyone at the company, and tells people who are self-employed, or whose plan is not available to everyone at the company, to ask their insurance provider whether it is employer group health plan coverage as the IRS defines it, and to sign up at 65 if it is not. You then have an 8-month Special Enrollment Period, and the sources state its start a month apart: CMS counts from the month employment or that coverage ends, whichever comes first, Social Security from the month after. Count from the earlier one. Coverage generally starts the first month after you sign up, and if you sign up while still working, or within the first full month after the employer coverage ends, you can ask to delay your Part B start date by up to 3 months. One group is carved out entirely: this Special Enrollment Period does not apply to people eligible for Medicare based on End-Stage Renal Disease (ESRD), who have a different route, covered below.

Retiree coverage: Medicare pays first

If you have coverage through a former employer's retiree plan, Medicare pays primary and the retiree plan pays secondary. This is true even if the retiree plan looks similar to, or has the same carrier as, the employer plan you had while working. When you become eligible for Medicare, you may need to sign up for both Part A and Part B to get full benefits from your retiree coverage, and that coverage might not pay your medical costs for any period when you were eligible for Medicare but didn't sign up.

That last point is what costs money. Retiree plans are built to wrap around Medicare, so without Medicare in place the secondary calculation can't be run and the plan may decline the claim entirely. The bill then lands on you.

Notify your retiree plan when you enroll in Medicare; plans often require advance notice to avoid billing errors.

The COBRA trap, explained in detail

COBRA is continuation coverage: when you leave a job that offered group health insurance, it lets you stay on that same plan temporarily if you pay the premiums yourself. It feels like a natural bridge into retirement, and that is the trap, because COBRA is not treated as "current employer coverage" for Medicare enrollment purposes.

You retire at 66, still covered by your employer's group plan, so your 8-month Special Enrollment Period starts the month your employment or that coverage ends, whichever comes first. Instead of enrolling in Part B, you elect COBRA, and because COBRA feels like coverage you assume the clock isn't running. It is. Per the Special Enrollment Period rules, the window started when your employment ended, not when COBRA expires, so waiting for COBRA to run out means very likely missing it entirely.

The penalty for missing Part B enrollment without qualifying for a Special Enrollment Period or a Medicare Savings Program, the two exceptions that apply to Part B, is a 10% increase in your Part B premium for each full 12-month period you could have had Part B and didn't, and per Medicare's late enrollment penalty rules, that surcharge lasts as long as you have Part B. The 2026 standard Part B premium is $202.90 a month, and that 10% rides on top of it for as long as you hold Part B, which for most people means the rest of their life.,

Missing that window closes the working-past-65 route, but it is not the end of the road, and the General Enrollment Period is not your only remaining option. If an employer, a health plan, or an agent or broker for one told you that you didn't need Part B, there is a Special Enrollment Period for exactly that: it begins the day you notify the Social Security Administration, ends six months later, and removes the wait and the penalty rather than merely delaying them. It is bounded, and you have to show both halves by documentation or written attestation, that you didn't enroll during an enrollment period you were eligible for based on what you were told, and that the employer, plan, or agent materially misrepresented or gave incorrect information about enrolling in Part B or premium Part A. Form CMS-10797 is the application. CMS lists six further Special Enrollment Periods alongside that one, including for a declared emergency or disaster, release from incarceration, and termination of Medicaid, and 42 CFR 406.24(d) preserves your right to another where a missed period was caused by group health plan coverage being restored before it ended. Only if none of them fits do you wait for the General Enrollment Period, January 1 through March 31, with coverage starting the first day of the month after you enroll.

If you're on COBRA and approaching Medicare age, enroll in Part B during your 8-month window from the end of employment. Don't wait for COBRA to run out.

COBRA coordination when you already have Medicare

The rules differ when Medicare is already in place before COBRA. If you're 65 or older and have Medicare because of your age, Medicare pays primary and COBRA pays secondary, the same structure as retiree coverage. COBRA still helps, covering cost-sharing Medicare leaves to you, but Medicare's primary role doesn't change.

Notify your COBRA administrator when you enroll in Medicare; most plans require it and will adjust how they process claims.

The ESRD exception: COBRA pays first for 30 months

There is one situation where COBRA pays primary instead of Medicare: when your Medicare eligibility is based solely on end-stage renal disease (ESRD), meaning permanent kidney failure requiring dialysis or a transplant. If that's you and you also have COBRA coverage, COBRA pays primary for a 30-month coordination period that begins on the date you are first eligible to enroll in Medicare because of ESRD. The same rule applies to any group health plan coverage, not just COBRA, regardless of employer size and regardless of whether anyone is still working, retirees included. After the coordination period ends, Medicare becomes the primary payer for all Medicare-covered services, and a separate 30-month coordination period runs each time you sign up for Medicare based on permanent kidney failure. The rule carries one precondition: it applies only if Medicare was not already your primary payer on the basis of age or disability at the time you became eligible for or entitled to Medicare on the basis of ESRD.

You don't have to do anything at that transition: Medicare moves into the primary position automatically. Still, confirm with your plan that the switch happened, so a claim isn't sent to the wrong payer.

This exception exists because Medicare covers ESRD at any age: no age or income test, though there is an insured-status test: you must have worked the required amount of time under Social Security, the Railroad Retirement Board, or as a government employee, or already be getting or be eligible for Social Security or Railroad Retirement benefits, or be the spouse or dependent child of someone who is.

One thing the 30-month coordination period does not do is reopen your Part B enrollment window. The working-past-65 Special Enrollment Period described earlier does not apply if you're eligible for Medicare based on End-Stage Renal Disease (ESRD), so if your Medicare eligibility comes from ESRD and you are also working past 65, the 8-month window after employer coverage ends is not yours to plan around.

That is not a penalty exposure, though, and acting as if it were is what costs money here. If you apply for Medicare and are approved because of ESRD, you can sign up for Part B without paying a late enrollment penalty. Medicare.gov goes further for people in the coordination period: if your group health plan will pay most or all of your health care costs, you may want to delay signing up until the 30-month coordination period is over, because delaying means not paying the Part B premium for coverage you don't need yet, and your Part B premium won't be higher because you delayed in this situation. Sign up once the coordination period ends, and sign up as soon as you can if the group health plan's benefits are cut or end during it. CMS's coordination manual says the same from the other side: people eligible on the basis of ESRD cannot use a Special Enrollment Period, but can defer both Part A and Part B and file an initial application later, usually at the end of the coordination period.

If your Medicare comes from both age and ESRD, don't assume which rule governs your claims. Confirm who pays first with the Benefits Coordination & Recovery Center, contact details below, before a large bill comes due.

Medicare coordination of benefits summary: who pays first

The table below gives the order for each situation, and the one thing to watch.

Your situation Primary payer Secondary payer Key note
Age 65+, actively working, non-tribal employer plan, 20+ employees Employer group plan Medicare Can delay Part B without penalty while employed
Age 65+, actively working, employer has fewer than 20 employees Medicare Employer plan Enroll in Medicare on time or employer plan may deny claims
Under 65, Medicare from disability, covered by current employment Employer group plan Medicare Only at 100+ employees; below that, Medicare pays first
Retired, have retiree coverage Medicare Retiree plan Retiree plan may deny claims if Medicare not enrolled
On COBRA, age 65+ (Medicare enrolled) Medicare COBRA COBRA does NOT extend your SEP; enroll in Part B during the 8-month window after employment ends
Medicare eligible because of ESRD only, also have COBRA COBRA Medicare COBRA pays first for the first 30 months (coordination period)

One caveat spans the last two rows: the 8-month Special Enrollment Period named in the COBRA row does not apply if you're eligible for Medicare based on End-Stage Renal Disease (ESRD). For ESRD the route is the one above: no late enrollment penalty, and waiting until the coordination period ends can be cheaper.

Telling your plans about each other

One practical step trips up more people than any rule: you have to tell your plans when your coverage changes. Nothing prompts you to do it, and each plan only knows what it has on file, so a change you consider obvious, like retiring or picking up Medicare, is invisible to your other insurer until you say so.

When you enroll in Medicare, tell your other insurer. When you retire and lose employer coverage, tell Medicare and any remaining plans. When you elect COBRA, tell that plan whether Medicare is already in place. A stale record is what produces a surprise denial months later.

Behind the scenes this runs through the Benefits Coordination & Recovery Center (BCRC), the CMS contractor that consolidates Medicare's coordination-of-benefits work. Section 111 of the MMSEA requires group health plans, and non-group plans such as liability, no-fault and workers' compensation insurers, to report to CMS so Medicare can coordinate benefits, so the record your plans work from is only as good as what has been reported. When your coverage is not on it yet, you may need to report the coverage yourself.

Two numbers are worth keeping on hand.

Benefits Coordination & Recovery Center (BCRC) The number Medicare gives for confirming which coverage pays first, alongside contacting your plan. 1-855-798-2627 (TTY 1-855-797-2627)
Medicare The official contact line for beneficiaries and CMS's route for an Original Medicare question, staffed 24 hours a day, 7 days a week, except some federal holidays. For who pays first, use the BCRC number above. 1-800-MEDICARE (1-800-633-4227), TTY 1-877-486-2048 medicare.gov coordination page

Frequently Asked Questions

Does COBRA count as current employer coverage for Medicare enrollment purposes?

No. Neither COBRA nor a retiree health plan counts as "current employer coverage" under Medicare's enrollment rules, so the end of either does not open this Special Enrollment Period and does not extend it. The 8-month window for Part B starts when your employment or your employer-sponsored coverage ends, whichever comes first, not when COBRA expires. Miss it without a Special Enrollment Period or a Medicare Savings Program and the Part B penalty is 10% added to your monthly premium for each full 12-month period you could have had Part B and didn't, for as long as you have Part B. Two things soften that. If you missed the window because your employer or your health plan told you that you didn't need Part B, check the misrepresentation Special Enrollment Period, which starts the day you notify Social Security and runs six months. And group health coverage based on severance or retirement pay is treated differently from COBRA: it can still open this window if the job it is based on ended within the last eight months.

How does a Medigap (Medicare Supplement) policy fit into coordination of benefits?

A Medigap policy isn't a competing primary or secondary payer; it's a supplement that sits behind Original Medicare, which always pays first. What it picks up after that depends on which lettered plan you bought, and the difference matters. Plan G, for example, pays the Part A deductible and coinsurance, the Part B coinsurance, skilled nursing facility and hospice coinsurance, the first 3 pints of blood, and Part B excess charges, but it does not pay the annual Part B deductible. The only two standardized plans that cover that deductible, C and F, are closed to people new to Medicare on or after January 1, 2020. Medigap also can't pay a Medicare Advantage plan's copayments, coinsurance, deductibles, or premiums, and while you're in an Advantage plan CMS says it is illegal for anyone to sell you a Medigap policy unless you're switching back to Original Medicare. That isn't a permanent lock-out: if you joined an Advantage plan for the first time, federal law gives you a trial right to buy a Medigap policy and a separate drug plan if you return to Original Medicare within 12 months of joining.

If my employer pays primary and Medicare pays secondary, do I need to enroll in Medicare?

If your employer has 20 or more employees, you can generally delay Part B without penalty, as long as the coverage is a group health plan based on your current employment and available to everyone at the company; the caveats for self-employment and plans not open to everyone are in the employer section above. The Special Enrollment Period that makes delaying safe also does not apply if you're eligible for Medicare based on End-Stage Renal Disease (ESRD). That does not mean an ESRD reader who delays gets penalized: someone approved for Medicare because of ESRD can sign up for Part B without a late enrollment penalty. You should still enroll in Part A if you qualify for it premium-free, which about 99% of beneficiaries do on at least 40 quarters of Medicare-covered employment, because there's usually no downside and it may pay something as a secondary payer. Part B can wait until you leave the job or lose the employer coverage.

What happens if I have a small employer (fewer than 20 employees) and don't enroll in Medicare?

Your employer plan is designed to pay secondary, because under 20 employees Medicare pays first and the group health plan pays second. If you haven't enrolled, the plan may calculate its liability assuming Medicare already paid its share and deny your claim for the remainder. Enrolling on time protects against this.

Can I have both Medicare Advantage and other coverage?

Yes. CMS states that while you're in a Medicare Advantage plan you may be able to use coverage from a current or former employer or union, or Medicaid if you qualify. Which plan pays first is still decided by the situations above. If you can't tell which is yours, confirm it with the Benefits Coordination & Recovery Center before a large bill comes due.

If I have retiree coverage and Medicare, do I still need to pay the Part B premium?

Yes. Part B carries its own monthly premium, $202.90 for the standard amount in 2026. Retiree plans coordinate with Medicare rather than replacing it: you may need both Part A and Part B for full retiree benefits, and that coverage might not pay your costs for any period when you were eligible for Medicare but didn't sign up. Dropping Part B to save the premium usually costs more than it saves.

Learn More

MedicareWhat is Medicare? Parts A, B, C, and D explainedWhat is Medicare? A plain guide to Parts A, B, C, and D, who qualifies, 2026 costs, when to enroll, and what it doesn't cover, with help-paying options. MedicareMedicare enrollment periods and late penaltiesWhen to sign up for Medicare without a lifelong penalty: the 7-month Initial Enrollment Period, working past 65, the GEP, and what each late penalty costs. MedicareCOBRA instead of Medicare Part B: can you wait?Thinking of keeping COBRA instead of Medicare Part B? It usually isn't safe: COBRA won't delay Part B and waiting can trigger a late-enrollment penalty. MedicareMedigap (Medicare Supplement): how the lettered plans workMedigap (Medicare Supplement) explained: how the standardized A-N plans work, Plan G vs Plan N, closed C and F plans, and the 6-month guaranteed-issue window. MedicareVA benefits and Medicare: how they work togetherHow VA benefits and Medicare work together: why veterans should take Part B at 65 to avoid a lifelong penalty, and delay Part D with VA drug coverage. MedicareTRICARE For Life and Medicare for military retireesTRICARE For Life coordinates with Medicare for military retirees, survivors and certain former spouses. Learn why Part B is required and who pays first. MedicareOriginal Medicare vs. Medicare AdvantageCompare original Medicare vs Medicare Advantage on cost, networks, the $9,250 cap, and switching back. A neutral 2026 side-by-side to help you choose. MedicaidMedicare Secondary Payer rules in GeorgiaGeorgia Medicare Secondary Payer (MSP) rules: when Medicare pays primary vs. secondary to employer plans, the 20-employee rule, COBRA trap, and Part B penalty.

Find personalized help coordinating Medicare with your other coverage at brevy.com.

The information on Brevy.com is for educational purposes only and is not a substitute for professional legal, financial, or medical advice. Rules vary by state and program and change frequently. Always verify with the relevant agency or a qualified professional. Brevy is not a law firm, financial advisor, or healthcare provider.

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