Medicare has several enrollment periods, each with its own dates, rules, and consequences for missing them. The one that applies to you depends on whether you’re signing up for the first time, changing an existing plan, or catching up after missing an earlier window. Understanding which period you’re in helps you avoid gaps in coverage and permanent penalties on your premiums.
Initial Enrollment Period
When you first become eligible for Medicare, typically at age 65, you get a seven-month window called the Initial Enrollment Period. It starts three months before the month you turn 65, includes your birthday month, and extends three months after it. If your 65th birthday is in June, for example, your window runs from March through September.
Signing up during the three months before your birthday month gives you the earliest possible coverage start date. If you wait until your birthday month or later within that window, your coverage start date gets pushed back. This matters if you’re leaving employer insurance and need seamless coverage.
If you’re already receiving Social Security benefits when you turn 65, you’ll be automatically enrolled in Medicare Parts A and B. Your Medicare card will arrive in the mail before your 65th birthday without you needing to do anything. If you’re not yet collecting Social Security, you need to actively sign up.
Open Enrollment: October 15 Through December 7
Open Enrollment (also called the Annual Enrollment Period) runs from October 15 through December 7 every year. This is the window when anyone already on Medicare can make changes to their coverage, with those changes taking effect January 1 of the following year. Your plan must receive your enrollment request by December 7.
During Open Enrollment, you can:
- Switch between Original Medicare and Medicare Advantage: move from one to the other in either direction.
- Change your Medicare Advantage Plan: join a new one, drop yours, or switch to a different plan with or without drug coverage.
- Change your drug plan: join, drop, or switch to a different Part D prescription drug plan if you’re in Original Medicare.
This is the most flexible enrollment period. If you’re happy with your current coverage, you don’t need to do anything. Your existing plan renews automatically for the next calendar year. Starting in 2026, CMS has finalized a new automatic renewal process for Part D enrollees that extends participation into the next year unless you actively opt out.
Medicare Advantage Open Enrollment Period
This is a separate, narrower window that runs from January 1 through March 31 each year. It’s only for people who are already enrolled in a Medicare Advantage plan. During this period, you can switch to a different Medicare Advantage plan or drop your Medicare Advantage plan and return to Original Medicare (and join a standalone Part D drug plan if needed).
You can only make one change during this period. It’s designed as a second chance if you enrolled in a Medicare Advantage plan during Open Enrollment and realized it isn’t working for you. If you’re on Original Medicare and not in a Medicare Advantage plan, this period doesn’t apply to you.
General Enrollment Period
If you missed your Initial Enrollment Period and didn’t sign up for Part A or Part B when you were first eligible, the General Enrollment Period gives you another chance. It runs from January 1 through March 31 each year. Coverage starts the month after you sign up.
This period exists as a safety net, but using it usually means you’ll face late enrollment penalties on your premiums (more on that below). It’s not a penalty-free do-over. The gap between when you were first eligible and when you actually enrolled determines how much extra you’ll pay.
Special Enrollment Periods
Certain life events open a Special Enrollment Period outside the standard windows. These let you enroll in or change your Medicare coverage without waiting for the next scheduled period. The most common qualifying events include:
- Loss of employer or union coverage: if you had insurance through a job (including COBRA) and that coverage ends, you qualify for an SEP to sign up for Medicare without penalty.
- Moving to a new area: if you relocate outside your current plan’s service area, or move somewhere that offers new plan options, you can switch plans.
- Loss of Medicaid eligibility: losing Medicaid coverage triggers an SEP.
- Leaving an institution: moving out of a nursing home or rehabilitation hospital qualifies you.
- Release from incarceration: you can enroll after being released from jail or prison.
- Plan problems: if Medicare sanctions your plan, your plan’s contract is terminated, or a state takes over your plan due to financial issues, you get an SEP to find new coverage.
The employer coverage SEP is especially important. Many people who work past 65 delay Medicare enrollment because they have group health insurance through their job. As long as you enroll within the SEP window after that coverage ends, you won’t face late penalties. This is one of the few situations where delaying Medicare is completely safe.
Late Enrollment Penalties
Missing your enrollment window doesn’t just delay your coverage. It can permanently increase your premiums. These penalties are not one-time fees. For most people, they last as long as you have that type of Medicare coverage, which typically means for life.
For Part B, you’ll pay an extra 10% on your monthly premium for each full 12-month period you were eligible but didn’t sign up. If you waited two years past your eligibility, that’s a 20% surcharge added to every monthly premium going forward. Ten years of delay means a 100% increase, and you’ll pay that higher amount permanently.
Part D penalties work differently but add up just as fast. You’ll pay an extra 1% of the “national base beneficiary premium” for each full month you went without creditable drug coverage. In 2026, that base premium is $38.99. So if you went 24 months without coverage, your penalty would be roughly $9.40 per month (24 times 1% of $38.99, rounded to the nearest ten cents), added to your premium for as long as you have Part D coverage. The penalty threshold kicks in after a gap of 63 days or more without creditable drug coverage.
Part A penalties apply only to people who have to pay a premium for Part A (most people get it premium-free based on their work history). If you’re required to buy Part A and don’t do so when first eligible, your premium goes up 10%, and you pay that higher rate for twice the number of years you delayed enrollment.
How to Enroll
The most straightforward path is through Social Security, since that agency handles Medicare enrollment. If you’re 65 or older, you can enroll online for Parts A and B, or Part A only, through the Social Security website. You also have the option to delay Part B online if you’re still covered through an employer group health plan.
If you prefer not to enroll online, you can call Social Security at 800-772-1213, available Monday through Friday from 8 a.m. to 7 p.m. in most U.S. time zones. Representatives are available in English, Spanish, and other languages. For people who are deaf or hard of hearing, the TTY number is 800-325-0778.
Enrolling in a Medicare Advantage plan or Part D drug plan is a separate step. After you have Original Medicare (Parts A and B), you can join these plans through the plan itself, through Medicare.gov’s plan finder tool, or by calling 1-800-MEDICARE. These plan enrollments happen during the enrollment periods described above.