The clock starts ticking the moment you’re eligible for Medicare, and missing those deadlines can add up to big costs over your lifetime.
While most people know Medicare helps with health care costs in retirement, many don’t realize that simple enrollment mistakes can trigger permanent financial penalties that compound year after year.
These aren’t minor fees, either. Here are Medicare mistakes that can threaten your retirement security.
1. Missing initial enrollment
Missing your initial enrollment period can trigger a late-enrollment penalty in the form of a higher premium for Part A (hospital insurance), Part B (outpatient care) or both.
With Medicare Part A, the late enrollment penalty can add an additional 10% to your monthly premium. You pay this penalty each month for twice the number of years that you didn’t sign up when you should have.
For each full year you delay Part B enrollment without creditable coverage, your monthly premium jumps by 10%. Just like Part D, this increase is permanent. Someone who waits three years past eligibility to sign up for Part B will pay a lifetime fee of 30% more than the standard premium.
2. Missing or skipping Part D enrollment
Skipping prescription drug, or Part D, coverage can haunt your wallet forever. The Part D late enrollment penalty adds 1% to your monthly premium for every month you go without creditable drug coverage after your initial enrollment period ends.
According to Medicare.gov, the national base beneficiary premium is $36.78 for 2025. If you wait 14 months to enroll, you’ll pay an extra 14% on top of your regular premium permanently.
3. Missing Medigap enrollment
Original Medicare, aka traditional Medicare, covers about 80% of many medical costs, leaving you responsible for the remaining 20%. That’s where a Medigap plan comes in. Medigap is private insurance that helps pay for expenses that Original Medicare does not cover.
The Medigap open enrollment period runs for six months from the first month you have Part B coverage and have selected Original Medicare during initial enrollment.
Only during your initial Medigap enrollment period are you guaranteed coverage by Medigap plans in your area. Then, and only then, are insurance companies forbidden from denying you coverage or charging you more money because of pre-existing conditions.
If you miss this enrollment period, an insurer won’t necessarily sell you a Medigap plan later, or you may have to pay a higher premium.
If you lose your initial Medigap coverage due to switching to Medicare Advantage — the type of Medicare offered by private insurers — you could end up paying significantly more for a Medigap policy if you later decide to switch back to Original Medicare. Or you could be barred from certain plans.
4. Switching Medicare Advantage plans
Medicare Advantage plans with low premiums might seem attractive, but the real costs often hide in the fine print. Networks change, benefits shift, and what works at 65 might leave you scrambling for coverage at 75.
The trouble comes when you want to switch back to Original Medicare. While you can change during annual enrollment, getting a Medigap plan to help with Original Medicare’s cost-sharing becomes difficult or impossible outside that initial window.
You could find yourself stuck choosing between inadequate network coverage or unaffordable out-of-pocket costs.
Protect yourself from Medicare mistakes
Avoiding these costly errors starts with understanding your enrollment timeline. Mark your calendar three months before turning 65, when your initial enrollment period begins. If you’re still working and have creditable coverage through an employer with 20 or more employees, you can delay certain parts of Medicare, but documentation is crucial.
Check your eligibility at Medicare.gov rather than assuming enrollment happens automatically. Only certain people receiving Social Security benefits get auto-enrolled. Everyone else needs to take action or face penalties.
Keep detailed records of any creditable coverage, whether from an employer, union, or military benefits. You’ll need proof of this coverage to avoid late enrollment penalties if you delay Medicare.
Consider working with a Medicare counselor through your State Health Insurance Assistance Program (SHIP). These free services help you navigate enrollment without sales pressure, potentially saving thousands in avoided mistakes.
Learn more in “13 Things That Are Free With Medicare.”
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