Retiring With Employer Coverage? Here’s How Medicare Coordination of Benefits Works

By Marc Gilman

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One of the most common retirement planning assumptions we encounter is this: “I’ll retire in July, I have employer coverage through the end of the month, and then I’ll figure out Medicare.” The problem isn’t that the person is wrong about the timeline — it’s that they often don’t realize how completely the relationship between their employer plan and Medicare changes the moment they stop working.

Understanding Medicare’s coordination of benefits rules — specifically, which plan pays first and which pays second — can be the difference between a clean, cost-effective transition and an expensive, confusing one.

Key Takeaways

• While actively employed at a company with 20 or more employees, your employer group health plan pays first. Medicare is secondary.

• The moment you retire, that relationship flips. Medicare becomes primary. Any former employer coverage — whether COBRA or retiree health insurance — pays secondary.

• COBRA is not the same as active employer coverage for Medicare purposes. From day one of COBRA, Medicare pays first.

• Average COBRA premiums in 2026 run $400–$700 per month for individual coverage — you’re now paying 102% of the full premium your employer was previously splitting with you.

• Medicare Part B + a Medigap supplement gives most retirees comprehensive coverage without needing COBRA, which is why keeping COBRA after retirement is usually redundant and expensive.

• COBRA may automatically terminate when Medicare begins — confirm this with your plan administrator before assuming both coverages will be active simultaneously.

• Don’t forget Part D. Employer coverage often included prescription drug coverage that was creditable. When you retire and that coverage ends, you need a Part D plan starting the same date as your Medicare, or you’ll face a late enrollment penalty.

How does employer coverage work with Medicare while you’re still working?

The federal rules that govern which plan pays first are called the Medicare Secondary Payer (MSP) provisions. For people over 65 who are actively employed:

• If your employer has 20 or more employees: Your employer group health plan pays first. Medicare is secondary. You can delay Medicare Part B enrollment without penalty as long as you remain actively employed and covered under that employer plan.

• If your employer has fewer than 20 employees: Medicare pays first. The employer plan is secondary. In this case, delaying Part B is generally a mistake because the employer plan may pay little or nothing without Medicare in place.

This distinction matters enormously. Most people working for mid-size or large companies are in the 20+ employee category — which means their employer plan has been primary throughout their working years and Medicare has been secondary (or they’ve simply deferred Part B entirely).

What changes the moment you retire?

Everything flips. The MSP rules tie primary payer status to active employment status — not just to having employer-connected coverage. The moment you retire and lose that active employment relationship, Medicare becomes the primary payer for all covered services.

This is true regardless of what type of coverage you carry after retirement:

• If you elect COBRA (continuation of your former employer’s group plan): Medicare is primary, COBRA is secondary — starting on day one of COBRA. COBRA is never the primary payer once Medicare is active.

• If your employer offers retiree health insurance: Medicare is primary, the retiree plan is secondary. Many retiree plans are specifically designed to wrap around Medicare — meaning they expect Medicare to pay first and they cover what Medicare doesn’t.

The rule is clean: active employment coverage = employer plan primary. Everything else = Medicare primary.

Here’s how it maps across the most common coverage situations:

If both coverages are active in the same month, which pays first?

Medicare pays first. The former employer plan — COBRA or retiree coverage — pays second. A Medigap supplement plan, if you have one, coordinates with Medicare and pays third (covering Medicare’s remaining cost-sharing).

In practice, with Medicare Part A + Part B + a Medigap Plan G:

1. Medicare Part A / Part B pays its share

2. Medigap picks up the remaining 20% coinsurance, deductibles, and excess charges

3. COBRA or retiree coverage is left with essentially nothing to pay

One important cost comparison that often gets overlooked: Original Medicare has no out-of-pocket maximum. The 20% Part B coinsurance applies without a ceiling, meaning a serious illness or major surgery could generate tens of thousands of dollars in Medicare cost-sharing with no cap. An employer plan with a $3,000 annual out-of-pocket maximum can look more cost-effective than Original Medicare on paper — until you add a Medigap plan, which eliminates most of that exposure. A Medigap Plan G covers the Part B deductible ($283 in 2026), the 20% coinsurance, and excess charges, giving you the equivalent of a capped out-of-pocket without the employer plan.

This is the core reason keeping COBRA after retirement rarely makes financial sense for someone who has Medicare and a Medigap plan. The employer coverage sits in third position behind two plans that together cover most costs. You’re paying the COBRA premium for coverage that provides little to no practical benefit.

What does COBRA actually cost in retirement — and is it worth it?

When you were employed, your employer was paying the majority of your health insurance premium — typically 70–80% of the total cost. You only saw the payroll deduction portion. COBRA ends that subsidy. You now pay 102% of the full premium (the entire employee plus employer share, plus a 2% administrative fee).

In 2026, average COBRA premiums for individual coverage run $400–$700 per month nationally. Northeastern states including New Hampshire tend to run toward the higher end of that range. Family coverage can exceed $1,500–$2,000 per month.

For a retiree with Medicare Part A + Part B + a Medigap plan, that COBRA premium buys coverage that coordinates behind Medicare and Medigap — meaning it rarely, if ever, pays anything. Compare that to:

• Medicare Part B premium in 2026: $202.90/month (standard rate; higher-income enrollees pay more under IRMAA)

• Medigap Plan G premium: varies by age, tobacco use, and carrier — typically $100–$200/month in New Hampshire for a 65-year-old — but provides comprehensive coverage with no network restrictions

Medicare Part B + a Medigap Plan G is almost always the more cost-effective and better-coordinated choice than Medicare + COBRA.

The COBRA trap most people miss

Before assuming you’ll have both COBRA and Medicare coverage simultaneously, check with your plan administrator. COBRA often terminates automatically when Medicare begins. Federal COBRA rules allow a plan to terminate continuation coverage when a qualified beneficiary becomes entitled to Medicare — and many plans do exactly this.

But the larger risk isn’t the overlap question — it’s the SEP clock. COBRA does not extend or pause your 8-month Special Enrollment Period. The SEP starts the month after your employment or employer coverage ends, regardless of whether you elect COBRA. This is the most expensive mistake retirees make.

If you spend those 8 months on COBRA and then drop it without enrolling in Part B, you have no SEP remaining. You must wait for Medicare’s General Enrollment Period — January 1 through March 31 each year — with coverage not starting until July 1. A permanent Part B penalty attaches for every 12-month period you were eligible but not enrolled. At the 2026 standard Part B premium of $202.90 per month, even a one-year delay adds over $20 per month — permanently, for life.

The practical example: someone retires June 30, elects COBRA July 1, and waits until the following March before trying to enroll in Part B. No SEP remains. They wait for the GEP, go months without Medicare coverage, and owe a permanent penalty from that point forward. The COBRA premiums paid during those months provided no protection against that outcome.

Don’t use COBRA as a transition plan into Medicare. Enroll in Part B during the 8-month SEP that begins the month after your employment ends.

What about retiree health insurance?

Some employers — particularly larger companies, government employers, and school systems — offer retiree health insurance separate from COBRA. This is a benefit, not a continuation right, and the terms vary by employer.

Retiree health plans almost always require Medicare to be in place. If you’re eligible for Medicare and don’t enroll, your retiree plan may refuse to pay secondary costs it would otherwise cover. Most retiree plans are designed to wrap around Medicare — they can’t run the secondary calculation without a primary Medicare payment. Failing to enroll in Medicare when eligible can leave you with no effective coverage even if you’re enrolled in the retiree plan.

If your employer offers retiree health insurance with good supplemental benefits — dental, vision, or prescription coverage not available elsewhere — it may be worth keeping alongside Medicare. But it should supplement Medicare, not replace it.

One specific trap with retiree drug coverage: If your former employer’s retiree plan includes prescription drug coverage, enrolling in a standalone Medicare Part D plan can cause the employer to cancel all retiree benefits — not just the drug coverage. Before adding a Part D plan to any coverage that includes a retiree plan, contact your benefits administrator in writing to confirm what changes. Saving $20 per month on a Part D plan is not worth losing dental, vision, and prescription coverage you’ve had for years.

The Part D gap most people overlook

Employer health coverage typically includes prescription drug coverage that qualifies as creditable — meaning it’s at least as good as Medicare Part D. While that creditable coverage is active through your employer, you can delay Part D enrollment without penalty.

The moment your employer coverage ends at retirement, that changes. You now have a window to enroll in Part D — and unlike the 8-month SEP for Part B, the Part D enrollment rules are stricter. If you go more than 63 consecutive days without creditable drug coverage, you’ll face a permanent late enrollment penalty of 1% of the national base beneficiary premium for every month you were without coverage, added to your Part D premium for as long as you have Medicare.

For a retiree leaving employer coverage July 31 with Medicare Part B starting August 1, a Part D plan should also start August 1. This is often the step that gets overlooked in transition planning.

The recommended transition timeline

Start at 64, not 65. By your 64th birthday, you should already know your employer’s size, what you currently pay in premiums, and what Medicare would cost with a Medigap or Medicare Advantage plan added. That comparison takes an hour with the right guidance — and if you wait until your coverage is ending, you’re making decisions under time pressure.

One practical note about billing: once you have both employer coverage and Medicare active, tell every provider, every time, which insurance pays first. Don’t assume the billing office knows. Claim denials due to incorrect billing order are common and create delays, paperwork, and temporary out-of-pocket charges while errors get corrected. Carry both insurance cards and present them at every appointment.

For a retiree leaving employer coverage on July 31 with Medicare Part B and Medigap effective August 1:

Before July 31:

• Confirm with HR in writing on company letterhead the exact date employer coverage terminates and whether the plan qualifies as current employment coverage under Medicare Secondary Payer rules — verbal confirmation from HR is not sufficient

• Confirm whether COBRA, if elected, would terminate upon Medicare becoming effective

• Submit Part B enrollment forms to Social Security at least one month before your intended Medicare start date — Part B coverage begins the first of the month after SSA receives your forms, so submitting in June produces an August 1 start date

• Apply for a Medigap plan — you have guaranteed-issue rights during your 6-month Medigap Open Enrollment Period, which runs from the month your Part B begins

• Select a Part D prescription drug plan for August 1 — you have a 2-month Special Enrollment Period from the date your employer drug coverage ends; don’t let it lapse past 63 days or a permanent penalty applies

July 31:

• Employer coverage ends

August 1:

• Medicare Part A active (most people have Part A from age 65; if not, confirm activation)

• Medicare Part B active

• Medigap plan active

• Part D plan active

No gap. No overlap. Clean transition.

Frequently Asked Questions

I’m retiring mid-month. Does my employer coverage end on my last day of work or the end of the month? Most group health plans terminate at the end of the last full month of employment, not on the last day worked. If your last day is July 15, coverage typically runs through July 31. Confirm this with your benefits administrator — it affects the start date you need for Medicare Part B and Medigap.

If my employer coverage is primary while I’m working, do I need to do anything when I retire to switch it to secondary? The coordination flip happens automatically under federal MSP rules when your employment status changes. However, you should notify your employer’s benefits administrator of your Medicare enrollment date, and notify Medicare of your retirement date. Proper coordination of benefits records in both systems prevents billing errors and claim delays.

My employer offers retiree health coverage — should I keep it or just use Medicare and Medigap? It depends on what the retiree plan covers. If it includes dental, vision, hearing, or drug coverage that isn’t available through Medicare alone, and the premium is reasonable, it may be worth keeping alongside Medicare. If it only duplicates what Medicare and Medigap already cover, the premium is a sunk cost. Compare benefits and costs carefully before committing.

What is the Part B late enrollment penalty if I miss the Special Enrollment Period? If you retire and don’t enroll in Part B within the 8-month Special Enrollment Period that begins when employment or employer coverage ends (whichever comes first), you’ll pay a 10% permanent penalty on the Part B premium for every 12-month period you were eligible but not enrolled. At the 2026 standard premium of $202.90 per month, a two-year delay adds approximately $40.58 per month — permanently.

Can I cancel employer coverage before my last day? Generally no — employer group health plan termination is tied to your last day of employment or the end of that month. What you can do is decline COBRA (by simply not enrolling within the 60-day election window) and let employer coverage end naturally on July 31.

Does this work the same way for Medicare Advantage instead of a Medigap supplement? Yes — the primary/secondary coordination rules apply whether you have Original Medicare + Medigap or Original Medicare replaced by a Medicare Advantage plan. Under Medicare Advantage, the Advantage plan steps into Medicare’s role as primary payer, and any former employer coverage (COBRA or retiree) remains secondary.

Transitioning from employer coverage to Medicare involves more moving parts than most people anticipate. If you’re retiring in the coming months and want to make sure your coverage lines up on the right dates without gaps, penalties, or redundant premiums, we’re glad to walk through it with you.

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