When you leave a job, you'll probably get a letter offering COBRA. It feels like the safe choice: same plan, same doctors. Many people assume they can stay on COBRA and sign up for Part B when it runs out.
That assumption can cost you for the rest of your life. Here's why, and what to do if you're already on COBRA.
What COBRA is, and what it isn't
COBRA is a federal law that may let you keep your employer's group health plan for a limited time after your job ends. It's generally offered for 18 months, and 36 months in some cases (Medicare.gov).
For Medicare, the key fact is this: COBRA is never coverage based on current employment (POMS HI 00805.330). That's true even when COBRA starts because your hours were cut rather than your job ending. So COBRA:
- doesn't qualify you for the Part B Special Enrollment Period,
- doesn't pause or extend your 8 months,
- doesn't open a new window when it ends, and
- doesn't protect those months from the late penalty.
Medicare.gov says it plainly: "Don't wait until your COBRA coverage ends to sign up for Part B. COBRA coverage doesn't extend your limited time to sign up for Medicare."
A worked example of the trap
Linda turned 65 in June 2024. She was still working and on her employer's plan, so she skipped Part B. That was fine. She retired, and her job and her employee coverage both ended in September 2025. She chose COBRA, which ran 18 months, from October 2025 through March 2027.
| When | What happened |
|---|---|
| September 2025 | Job and employee coverage end. This month starts the clock. |
| October 2025 | COBRA begins. Her 8-month Part B window opens. |
| May 31, 2026 | Last day to sign up for Part B without a penalty. COBRA still has 10 months to go. |
| March 2027 | COBRA is ending. She signs up during the General Enrollment Period. |
| April 1, 2027 | Part B starts, with a late penalty. |
Her penalty: the months from October 2025 through March 2027 count, because COBRA isn't current-employment coverage. That's 18 months, which is one full 12-month period, so the penalty is 10%. At the 2026 standard premium of $202.90, that's $202.90 + $20.29 = $223.19, rounded to $223.20 a month. She'll pay the 10% extra for as long as she has Part B.
The hidden cost: Medicare.gov warns that if you have COBRA and are eligible for Medicare but not enrolled, COBRA may pay for only a small part of your care. Linda may have been paying for COBRA that wasn't paying much.
It could have been worse. If her COBRA had run out in the summer, she couldn't have signed up until the next January, with months of no coverage at all.
What she should have done: sign up while still working, say in July or August 2025, and pick October 2025 as her Part B start month. Part B would have started the month her employee coverage stopped. See when does Part B start.
Check your Part B Special Enrollment Period in 2 minutes. Start the free Part B check →
Already on COBRA? Check where you stand
- Find the last month you worked and the last month of your employee coverage (not COBRA). Take the earlier one.
- Your 8 months start the month after that. For example, if your job ended in December 2026, your window is January through August 2027.
- Compare that to today.
If you're still inside your 8 months
Sign up now. Send CMS-40B with CMS-L564. Section B of the L564 shows when your employee coverage ended, which is what matters. Ask your COBRA plan what happens to it once you have Medicare.
If your 8 months are over
- You can sign up in the General Enrollment Period, January 1 to March 31. Part B starts the month after you sign up.
- You may owe a late penalty. Months on your employer's plan while you were working are left out of the count, but COBRA months are not. See how the penalty is counted.
If someone told you to take COBRA instead of Part B
There's an "exceptional conditions" Special Enrollment Period for people who missed a Medicare window because their employer, health plan, or a plan's agent or broker gave them wrong information on or after January 1, 2023 (42 CFR 407.23).
Having COBRA isn't enough by itself
CMS said it "will not consider enrolling in COBRA alone as a basis for this SEP" (87 FR 66454). But if you were wrongly told to enroll in COBRA, you can give documented evidence or a written statement of the misinformation to qualify.
- The period starts the day you tell Social Security about the misinformation and lasts 6 months.
- Part B starts the month after you enroll.
- You apply with form CMS-10797, sent to your local Social Security office.
- There's no late penalty when you enroll this way (POMS HI 01001.010).
A separate "other exceptional conditions" period exists for events truly outside your control. Social Security says it won't be granted for forgetfulness or not knowing the rules.
Who to ask
- Your former employer's benefits administrator, about your COBRA options.
- Your State Health Insurance Assistance Program (SHIP), for free, unbiased help. Find yours at shiphelp.org.
- The Benefits Coordination & Recovery Center, 1-855-798-2627 (TTY 1-855-797-2627), for questions about COBRA and Medicare.
For the full rules on the 8-month window, see the Part B Special Enrollment Period guide.