Medicare Enrollment Periods: When and How to Sign Up

Getting Medicare enrollment timing right is one of the most financially consequential decisions you’ll make around retirement. Enroll at the right time and your coverage starts smoothly. Miss the wrong window and you could face a permanent premium penalty — and a gap in coverage that exposes you to significant out-of-pocket risk.

This guide explains every Medicare enrollment period, how they work, and the most common mistakes people make.

The Four Core Medicare Enrollment Periods

1. Initial Enrollment Period (IEP)

Your first opportunity to enroll in Medicare is the Initial Enrollment Period (IEP) — a 7-month window surrounding your 65th birthday:

• 3 months before the month you turn 65
• The month you turn 65
• 3 months after the month you turn 65

When your coverage starts depends on when you enroll within this window:

Enrolling early — in the first three months of your IEP — gets you the fastest start. Waiting until the month of or after your birthday delays coverage.

What you can enroll in during the IEP: Medicare Part A, Part B, a standalone Part D plan, and Medicare Advantage (Part C) plans.

2. General Enrollment Period (GEP)

If you miss your IEP without qualifying for a Special Enrollment Period, your next opportunity is the General Enrollment Period (GEP): January 1 through March 31 each year, with coverage starting July 1 of that year.

The GEP comes with a downside: if you needed Part B and missed your IEP without a qualifying exception, you’ll owe the Part B late-enrollment penalty. This permanent surcharge adds 10% to your Part B premium for each full 12-month period you were eligible but didn’t enroll.

Example: You were eligible at 65 in 2023 but didn’t enroll. You sign up during the January–March 2025 GEP. That’s two full years of delay — your Part B premium increases by 20% permanently.

At $185/month (2025 standard premium), a 20% increase means paying an extra $37/month for life — over $440/year, and compounding as base premiums rise.

3. Special Enrollment Period (SEP)

A Special Enrollment Period lets you enroll in Medicare outside the IEP and GEP without triggering a late-enrollment penalty. The most common qualifying event is continued employer coverage.

Active employer coverage SEP: If you’re working past 65 and covered by a group health plan through your own active employment (or a spouse’s active employment), you can delay Part B and Part D without penalty. You have an 8-month SEP starting the month after your employment ends or your group coverage ends — whichever comes first.

Critical rules for the employer coverage SEP:

• Coverage must be from active employment — not COBRA, retiree coverage, or marketplace coverage
• COBRA does not qualify you for an SEP — if you rely on COBRA to bridge to Medicare, you may face a late penalty
• The SEP is 8 months — not until the next General Enrollment Period

Other qualifying SEP events include:

• Moving to a new service area
• Losing Medicaid or Extra Help eligibility
• Gaining creditable drug coverage
• Certain changes in Medicare Advantage or Part D plan availability

4. Annual Enrollment Period (AEP)

The Annual Enrollment Period runs from October 15 to December 7 each year. This is not an enrollment period for first-time Medicare enrollees — it’s for people who are already on Medicare and want to change their coverage.

During the AEP, you can:

• Switch from Original Medicare to Medicare Advantage
• Switch from Medicare Advantage to Original Medicare
• Switch between Medicare Advantage plans
• Join a Part D plan (if you don’t have one)
• Switch between Part D plans
• Drop Part D coverage

Changes made during AEP take effect January 1 of the following year.

The AEP is your annual opportunity to review your coverage. Part D drug formularies and premiums change each year, as do Medicare Advantage plan networks and benefits. Even if you’ve been happy with your current plan, it’s worth comparing options each fall.

Medicare Advantage-Specific Enrollment Periods

Medicare Advantage Open Enrollment Period (OEP)

From January 1 to March 31 each year, Medicare Advantage enrollees have an additional window to:

• Switch from one Medicare Advantage plan to another
• Drop Medicare Advantage and return to Original Medicare (adding a Part D plan if desired)

This period is only available if you’re already enrolled in a Medicare Advantage plan. You cannot use it to enroll in Medicare Advantage for the first time — you must use the IEP or AEP for initial Advantage enrollment.

Initial Coverage Election Period for Medicare Advantage

When you first become eligible for Medicare, you can enroll in a Medicare Advantage plan during your IEP. If you’re already enrolled in Original Medicare, you can switch to Medicare Advantage during the AEP or if you have a qualifying SEP.

The Part D Late-Enrollment Penalty

The Part D late-enrollment penalty applies separately from the Part B penalty and is calculated differently.

The formula: 1% of the national base beneficiary premium × number of full months you were eligible for Part D but didn’t have creditable coverage.

In 2025, the national base beneficiary premium is $34.70. So each month of uncovered delay costs you about $0.35/month — permanently added to your Part D premium.

Example: You delay Part D for 30 months (2.5 years). Your penalty is 30% × $34.70 = $10.41/month — added to your Part D premium forever.

What Is Creditable Coverage?

You avoid the Part D penalty if you have “creditable” prescription drug coverage — coverage at least as good as standard Part D. This includes:

• Employer or union group health plan drug coverage (must notify you annually if it’s creditable)
• TRICARE
• VA prescription drug benefits
• Retiree drug coverage from a former employer (if certified as creditable)
• CHIP drug coverage

Keep documentation showing you had creditable coverage. If you’re ever asked to prove it, you’ll need the annual “creditable coverage” notice your insurer should have sent you.

Common Enrollment Mistakes and How to Avoid Them

Mistake 1: Relying on COBRA to Delay Medicare

COBRA continuation coverage extends your employer health plan temporarily after you leave a job — but it doesn’t count as “active employer coverage” for Medicare purposes. If you retire at 65, take COBRA, and skip Medicare, you’ll face the Part B (and possibly Part D) late-enrollment penalty when your COBRA runs out.

Solution: Enroll in Medicare Parts A and B when you retire, even if you elect COBRA for other coverage.

Mistake 2: Missing the 8-Month Window After Leaving Work

If you work past 65 with active employer coverage and then retire, you have only 8 months to enroll in Part B without penalty. Many people believe they can wait until the next annual enrollment period — they cannot. Missing the 8-month SEP window means waiting for the General Enrollment Period (January–March) with a penalty attached.

Solution: Enroll in Part B within 8 months of your employment or employer coverage ending.

Mistake 3: Confusing Observation Status With an Inpatient Stay

This affects SNF coverage, not enrollment — but it’s a costly mistake that often arises during the Part A benefit period. See Medicare Part A coverage for details.

Mistake 4: Not Enrolling in Part A When It’s Free

Most people are entitled to premium-free Part A (if they’ve worked 40+ quarters). There’s no reason to delay. Even if you have employer coverage, enrolling in Part A early means it can act as secondary coverage.

One exception: If you contribute to a Health Savings Account (HSA), enrolling in Part A (or Part B) makes you ineligible for future HSA contributions. If you’re actively contributing to an HSA and have good employer coverage, delay Medicare enrollment until you stop contributing.

Mistake 5: Forgetting to Compare Plans Each October

Your Medicare coverage doesn’t automatically optimize itself. Drug formularies change, plan premiums increase, and your health needs evolve. The AEP (October 15–December 7) is your annual window to check whether a different plan serves you better.

Solution: Each fall, use Medicare’s Plan Finder at Medicare.gov to compare your current plan against alternatives based on your current medications and providers.

Mistake 6: Not Understanding Medigap Open Enrollment

Medigap has its own enrollment window: a 6-month period starting when you turn 65 and are enrolled in Part B. During this window, insurers cannot deny you a Medigap policy or charge more due to pre-existing conditions.

After this window, you generally have no guaranteed right to buy Medigap. If you develop health conditions later and want to switch, you may find coverage unavailable or unaffordable.

Solution: If you want Medigap coverage, purchase it during your Medigap open enrollment window. See Medigap plans compared for help choosing the right plan.

Medicare and Employer Coverage: Working Past 65

If you continue working past 65, the interaction between your employer health plan and Medicare depends on your employer’s size:

Employer with 20+ employees: Your employer health plan is the primary payer; Medicare is secondary. You can often delay Part B without penalty as long as you have active employer coverage.

Employer with fewer than 20 employees: Medicare is the primary payer. If you don’t enroll in Part B, your employer plan may deny claims because it expects Medicare to pay first. Enrolling in Part B is important in this situation.

Self-employed: Your self-employed health coverage (marketplace plan, small group plan) typically doesn’t count as employer group coverage for Medicare purposes. Delaying Medicare enrollment while on a marketplace plan may trigger a late penalty.

Enrolling in Medicare: The Mechanics

To enroll in Medicare:

• Online: Visit SSA.gov and sign in to my Social Security to enroll during your IEP
• By phone: Call 1-800-772-1213 (SSA) — available Monday through Friday
• In person: Visit your local Social Security Administration office

If you’re already receiving Social Security retirement benefits, you’re typically enrolled in Medicare Parts A and B automatically at 65. You’ll receive your Medicare card in the mail about 3 months before your birthday.

If you’re not yet collecting Social Security, you need to actively enroll during your IEP. It’s not automatic.

Key Takeaways

• Your Initial Enrollment Period is a 7-month window around your 65th birthday — enroll during the first 3 months for the fastest coverage start
• The Part B late-enrollment penalty is 10% per year of delay, permanent
• The Part D late-enrollment penalty is 1% per full month of delay, permanent
• COBRA does not count as active employer coverage — don’t rely on it to delay Medicare
• After leaving employer coverage, you have 8 months to enroll in Part B penalty-free
• The Annual Enrollment Period (October 15–December 7) is your window to review and change plans each year

For help understanding what each part of Medicare covers, see Medicare Part A and Medicare Part B. If you’re deciding between Original Medicare and Medicare Advantage, compare the two approaches here.