When you leave a job at or after 65, you are typically offered COBRA continuation coverage — the option to stay on your former employer's health plan for up to 18 months by paying the full premium yourself. It looks like your employer coverage. It uses the same insurance card. The benefits are identical. For Medicare purposes, it is completely different. And the difference can cost you a lifetime of higher premiums.
The Medicare Special Enrollment Period — the protected eight-month window that lets you enroll without penalty after leaving employer coverage — applies only when that coverage was based on active employment. Retiree health plans do not qualify. COBRA does not qualify. Both of these continue your insurance after you stop working, which is exactly why people assume they are equivalent. They are not equivalent. Active employment means you are currently working. Coverage continuation products are what you use after you stop working. Medicare draws a hard line between the two.
The COBRA trap has a second layer that operates immediately, not just at the end of the 18-month coverage period. When a person is eligible for Medicare and also on COBRA, Medicare is the primary payer and COBRA is the secondary payer under federal coordination-of-benefits rules. This is true from the first day of Medicare eligibility — meaning the first day of the month you turn 65, if you have not taken other steps to delay Medicare. If you choose COBRA at 65 and do not enroll in Medicare, here is what happens in practice: a medical claim is submitted to COBRA. COBRA, knowing it is legally secondary, calculates what Medicare would have paid and removes that amount from its payment. You receive a fraction of the benefit you expected — or nothing — because your primary insurance is Medicare, and you do not have it.
Here is how it typically unfolds. A person retires at 65. They are offered COBRA. COBRA seems familiar and lower-friction than navigating Medicare, so they take it. Eighteen months later, COBRA ends. They call to enroll in Medicare. They are told they are outside the Special Enrollment Period. They must wait for the General Enrollment Period (January through March), with coverage starting the following month — potentially adding another four to six months without coverage. And they owe a permanent 10% per year Part B penalty for every 12-month period they delayed without qualifying coverage. In the 18-month COBRA scenario, that is one full 12-month delay period, producing a permanent 10% premium surcharge. On the 2026 Part B premium of $202.90, that is $20.29 extra per month for life — and rising every year the base premium increases.
The correct sequence when retiring at 65 is to enroll in Medicare Parts A and B during the Special Enrollment Period triggered by leaving active employer coverage — not to use COBRA as a Medicare substitute. The SEP begins the month after active employment or Group Health Plan coverage ends, whichever comes first, and lasts for eight months. If there is a brief gap between employer coverage ending and Medicare beginning — which can happen depending on timing — a very short COBRA bridge is sometimes used to cover that specific gap. This is different from using COBRA as a long-term alternative to Medicare enrollment. For Part D, the same principles apply: COBRA drug coverage is not creditable in the same way active employer coverage is. Enrolling in a Part D plan during the SEP avoids the monthly 1% penalty that would otherwise begin accruing after 63 days without drug coverage.
COBRA is not "creditable coverage" that delays your Medicare|Medicare, you will have been outside your SEP for 18 months. You will|General Enrollment Period for coverage to begin.|If you are on COBRA after turning 65 and have not enrolled in