Medicare and COBRA Rules: Why COBRA Can Cost You Thousands

COBRA feels like a safety net. You retire or lose your job, and COBRA lets you keep your employer health insurance for up to 18 months. It seems like a logical bridge to Medicare — except Medicare doesn't see it that way, and the consequences of that misunderstanding are permanent.

Federal rules are unambiguous: COBRA does not count as coverage based on current, active employment. The same applies to retiree health benefits. Neither qualifies you for a Special Enrollment Period, and neither protects you from late enrollment penalties.

The COBRA Trap Explained

When you leave a job with group health insurance after age 65, two things happen simultaneously:

1. Your 8-month Special Enrollment Period (SEP) starts — this clock begins when your active employment or group coverage ends, whichever comes first
2. You become eligible for COBRA — giving you up to 18 months of continued coverage under your former employer's plan

Here's the trap: COBRA lasts 18 months, but your Medicare SEP lasts only 8 months. If you stay on COBRA without enrolling in Part B, you'll blow past your penalty-free enrollment window by 10 months.

When the SEP expires, you must wait for the next General Enrollment Period (January 1 through March 31). Coverage won't start until the month after you enroll — potentially leaving you completely uncovered for months.

The Penalty Math

The Part B late enrollment penalty is calculated as 10% of the standard premium for every full 12-month period you were eligible but didn't enroll. The 2026 standard Part B premium is $202.90 per month.

An 18-month COBRA period without Part B enrollment creates a 10-month gap beyond the 8-month SEP. That's not a full 12-month period, so the penalty initially looks like it might not apply. But factor in the months waiting for the next GEP (which opens only in January through March), and the gap easily stretches past 12 months.

For a 24-month total delay, the permanent surcharge is 20% of $202.90 = $40.58 per month, added to every Part B premium payment for life.

COBRA and Claims: The Secondary Payer Problem

Even while you're on COBRA past 65, there's an immediate financial risk most people don't know about.

Under Medicare Secondary Payer rules, COBRA is secondary to Medicare once you're eligible. This means:

• COBRA pays claims as if Medicare were already active
• If you don't have Part B, COBRA covers only the portion it would have paid as a secondary insurer
• You're personally responsible for the remaining 80% of outpatient and physician costs

A single ER visit followed by outpatient imaging and specialist consultations can easily generate $15,000 in charges. Without Part B, COBRA might cover $3,000 of that. You owe the rest.

Retiree Health Insurance: Same Rules Apply

Retiree health benefits — coverage your former employer offers as part of a retirement package — follow the same rules as COBRA. They are not active employer coverage, and they do not extend your Special Enrollment Period.

The distinction matters: active employer coverage means you (or your spouse) are currently working for the employer providing the insurance. The moment active employment ends, the coverage stops being "active" regardless of what the plan is called or how long it continues.

Some retiree plans explicitly require Medicare enrollment at 65 and function as Medicare supplements. Others don't mention Medicare at all, leaving retirees to assume they're fully covered. Always check with your former employer's benefits office to understand how the retiree plan coordinates with Medicare.

What You Should Do Instead

If you're retiring at 65 or older:

1. Enroll in Part B during your Special Enrollment Period — start the paperwork 60 days before your last day of work using Forms CMS-L564 and CMS-40B
2. Use COBRA as a bridge only for coverage types Part B doesn't provide (dental, vision, hearing) or to maintain group rates while you shop for Medigap
3. Enroll in Part D within 63 days of losing your employer's creditable drug coverage

If you're already on COBRA past 65 without Part B: Check whether your SEP window is still open (8 months from when active employment ended). If it is, enroll immediately. If it has closed, you'll need to wait for the General Enrollment Period (January through March), and the late penalty will apply.

If you're on retiree health coverage: Verify whether the plan coordinates with Medicare. If it pays secondary to Medicare, you need Part B to avoid claim denials. If the plan will drop you at 65 without Medicare enrollment, you absolutely must enroll during your IEP.

Get the Complete Enrollment Toolkit

The Medicare Enrollment Guide includes a COBRA-to-Medicare transition timeline, the exact CMS-L564 and CMS-40B workflow for avoiding gaps, and penalty calculators showing the lifetime cost of delayed enrollment. It walks through every scenario where COBRA and retiree coverage intersect with Medicare deadlines.

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