Even if you are eligible for Medicare coverage, you can’t simply enroll whenever you want. Medicare enrollment is restricted to specific enrollment periods.

The Medicare Special Enrollment Period, or SEP, applies to situations where you need to enroll in Medicare or change your coverage outside of the standard enrollment periods. Most SEPs last for 60 to 90 days and can be triggered by a wide variety of events.

What types of enrollment periods does Medicare use?

Medicare's standard enrollment periods are:

• Initial Enrollment Period (IEP): Your Initial Enrollment Period begins three months before your Medicare eligibility date. If you age into Medicare, that means three months before the month of your 65th birthday. If you qualify due to disability, your IEP begins in month 22. The exception is those who qualify due to ALS or ESRD and who don't have the 2-year waiting period to apply for Medicare. In that case, your IEP begins the day your Medicare coverage does. In every case, the IEP lasts for seven months. During your IEP, you may sign up for Part A and/or Part B, a Medicare Advantage plan (Part C), Medicare prescription drug coverage (Part D), and/or Medicare Supplement Insurance (better known as Medigap).
• Medigap Open Enrollment Period (OEP): For the first six months that you have both Medicare Part B and are age 65 or older, you can apply for a Medicare Supplement plan without undergoing medical underwriting, being denied coverage due to pre-existing conditions, or charged a higher premium for health reasons. Outside of your Medigap OEP, your application usually goes through medical underwriting, which can result in the insurance company denying you coverage or charging a higher rate.
• Annual Enrollment Period (AEP): Everyone currently enrolled in Medicare can make changes to their coverage during Annual Enrollment, which lasts from October 15 through December 7.
• Medicare Advantage Open Enrollment Period (OEP): If you currently have a Medicare Advantage plan, you may either switch to a new plan or return to Original Medicare during OEP, which lasts from January 1 through March 31. If making that change causes you to lose your prescription drug coverage, you may also join a standalone Part D plan at that time.
• General Enrollment Period (GEP): From January 1 through March 31, anyone who missed their Initial Enrollment Period can sign up for Original Medicare. You then have from April 1 through June 30 to choose a Part D prescription drug plan and/or a Medicare Advantage plan. All coverage choices will begin on July 1.

Special Enrollment Periods are available to people experiencing what Medicare calls special circumstances, such as losing your employer coverage or moving. The full list is available on Medicare.gov here.

The Medicare Part B Special Enrollment Period is available to those who delay signing up for Part B because they have coverage through an employer (theirs or their spouse's). Once that coverage ends, they qualify for an SEP that lasts for eight months. It begins either the day coverage ends or the day employment ends – whichever comes first.

Please note that, to qualify for the Part B SEP, you must have been actively employed. Health insurance through a retirement plan or COBRA does not qualify as you are not "actively" employed. The same is true if the insurance is through a spouse's employment. Once you or your spouse is no longer actively employed, the Part B Special Enrollment Period begins. Full details are available from the Social Security Administration (SSA) here.

How is the Part B Special Enrollment Period different?

The Part B Special Enrollment Period functions uniquely compared to other enrollment periods. It is usually invoked when you deferred your Original Medicare enrollment because you were already covered through your employer. Assuming your company employs at least 20 people, Medicare will allow you to keep your employer-based coverage and then switch to Medicare later with no penalty. (Group health insurance through a company with fewer than 20 employees does not qualify as "creditable" coverage. Please see our article, "What Is Medicare Creditable Coverage?")

This is distinct from other uses of the SEP, such as moving out of the coverage area of your Medicare Advantage plan. However, the basic principle of Special Enrollment is the same.

Can I receive a late enrollment penalty?

If you defer Original Medicare coverage without being covered by an employer-based plan, you may have a late enrollment penalty when you do enroll in Original Medicare. This penalty amounts to 10% of what you would pay each month for every 12-month period when you could have had Medicare Part B but didn’t.

This means that if you wait two full years before you enroll in Medicare without having creditable coverage elsewhere, you could pay a 20% higher premium every month. This penalty lasts for the duration of your coverage, so it’s very important to avoid it if possible.

Can I enroll while I’m still covered by my employer-based plan?

Yes, you can enroll in Medicare Part B even though you are still enrolled in your employer-based plan. You will be able to enroll in Medicare at any time, and you won’t face any penalties. Which plan is considered primary and which is secondary depends on the size of your employer.

• If the company employs 20 or more people, Medicare is secondary and your employer plan is the primary payer.
• If the company employs fewer than 20 people, Medicare is the primary payer. It's very important you sign up for Medicare during your IEP to avoid lifelong late enrollment penalties.