Medicare Doesn’t Cover Everything. These Gaps Can Become Expensive Fast
Many people reach 65 believing Medicare will take over most of their health-care expenses. Medicare provides substantial coverage, but Original Medicare Parts A and B were never designed to pay 100% of every medical bill. Deductibles, coinsurance, prescription drugs, dental care and other gaps can leave retirees with considerably more financial exposure than they expected.
The biggest surprise is that Original Medicare has no annual limit on what a beneficiary can spend out of pocket for covered Part A and Part B services unless another form of coverage is added. Someone with expensive outpatient treatment could continue owing their share of Medicare-approved costs throughout the year. That makes choosing between Original Medicare with supplemental coverage and Medicare Advantage one of the most consequential decisions a new beneficiary makes.
Original Medicare Leaves Real Gaps
Medicare Part A primarily covers inpatient hospital care, while Part B generally covers doctors, outpatient services, medical equipment and many other medically necessary services. Both programs require beneficiaries to share costs, and Original Medicare does not include the type of annual out-of-pocket maximum common in many employer plans. (medicare.gov)
For 2026, the Part A inpatient hospital deductible is $1,736. Unlike a typical annual deductible, it applies to each benefit period, meaning someone can potentially owe it more than once during the same calendar year if separate hospital benefit periods occur. Hospital stays beyond 60 days can also trigger daily coinsurance amounts of $434 for days 61 through 90 and $868 for lifetime reserve days. (cms.gov)
Part B creates a different type of exposure. After the $283 annual Part B deductible in 2026, beneficiaries generally pay 20% of the Medicare-approved amount for covered Part B services, and Original Medicare does not place a yearly ceiling on that liability. (medicare.gov)
The 20% Coinsurance Can Become the Bigger Risk
A 20% coinsurance rate may not sound particularly threatening when a doctor’s visit costs a few hundred dollars. It becomes much more significant when treatment involves chemotherapy, outpatient surgery, expensive medical equipment or repeated specialist care.
If Medicare approves $50,000 of Part B-covered care, the beneficiary’s share could potentially be $10,000 before considering any additional coverage or other cost-sharing rules. Because Original Medicare itself does not impose an annual out-of-pocket limit, a second expensive episode later in the year can create additional liability.
That is why looking only at monthly Medicare premiums can be misleading. The more important financial question is how much exposure remains if someone experiences a serious illness rather than an average year.
Medigap Can Make Original Medicare More Predictable
Medicare Supplement Insurance, commonly called Medigap, is designed to help cover some of the deductibles and coinsurance left by Original Medicare. Standardized plans differ in what they cover, but many beneficiaries use Medigap specifically to make medical expenses more predictable. (medicare.gov)
The trade-off is paying an additional monthly premium even during healthy years. Someone may go months without needing significant medical care while continuing to pay the Medigap premium, which can make the coverage seem expensive when viewed only through short-term usage.
The value becomes clearer during years with substantial medical treatment. Instead of accepting potentially large Original Medicare cost sharing, the beneficiary transfers much of that risk to the supplemental insurer in exchange for the premium.
Medicare Advantage Controls Risk Differently
Medicare Advantage provides another way to receive Part A and Part B benefits through a private Medicare-approved plan. These plans must cover the medically necessary services Original Medicare covers, and most also include Part D prescription coverage and may offer additional benefits such as dental, hearing or vision services. (medicare.gov)
One major financial difference is that Medicare Advantage plans have an annual limit on what beneficiaries pay for covered Medicare Part A and Part B services. Once the plan’s applicable limit is reached, the member generally pays nothing further for covered services for the remainder of that year. (medicare.gov)
That protection comes with other trade-offs. Medicare Advantage plans may use provider networks, different copayments and coinsurance, and prior authorization for certain services, while Original Medicare generally allows beneficiaries to see any provider nationwide that accepts Medicare. A low or $0 plan premium therefore does not necessarily mean lower total costs for someone who becomes seriously ill.
Starting Cheap Can Be More Complicated Than It Looks
Some beneficiaries consider beginning with a low-premium Medicare Advantage plan and switching to Original Medicare with Medigap later if their health changes. The difficulty is that Medigap enrollment protections do not operate like Medicare Advantage enrollment rules.
The federal Medigap Open Enrollment Period generally lasts six months beginning when someone is 65 or older and first enrolled in Part B. During that period, insurers cannot use medical underwriting to deny coverage or charge more because of health problems. Outside that protected period, federal guaranteed-issue rights apply only in certain circumstances, although some states provide additional protections.
That means a healthy beneficiary may find Medicare Advantage inexpensive today but should still understand whether obtaining a Medigap policy later could require underwriting. Waiting until a major diagnosis occurs to investigate the rules can leave fewer options than expected.
State “Birthday Rules” Are Not Nationwide
Some states have enacted Medigap birthday rules that allow existing Medigap policyholders to make certain changes around their birthdays without medical underwriting. Those protections can help beneficiaries shop for lower premiums or different carriers, but the rules vary substantially by state.
There is no nationwide Medicare birthday rule allowing everyone to switch Medigap plans freely each year. The permitted window, eligible plans and switching restrictions depend on state law, so beneficiaries should not assume advice that applies in one state applies everywhere.
This distinction matters because Medicare marketing frequently uses broad phrases such as “open enrollment” without explaining which type of coverage is actually involved. Medicare Advantage, Part D and Medigap all follow different enrollment and switching rules.
Part D Has Its Own Penalty Trap
Prescription-drug coverage creates another common source of confusion. Medicare Part D is optional, but someone who goes 63 consecutive days or more after their eligible enrollment period without Part D or other creditable prescription-drug coverage may owe a late enrollment penalty if they later enroll. (medicare.gov)
For 2026, the penalty is calculated using 1% of the $38.99 national base beneficiary premium for each full uncovered month. The resulting amount is added to the beneficiary’s Part D premium and generally continues for as long as the person has Medicare drug coverage. (medicare.gov)
That makes documentation important for people delaying Part D because they had employer coverage. Employer or union plans are required to tell participants whether their prescription coverage is creditable, and beneficiaries should keep those notices in case Medicare or a future Part D plan asks for proof.
Employer Coverage Can Delay Medicare—but the Rules Depend on the Employer
Working past 65 does not automatically mean someone must enroll in every part of Medicare immediately. People covered by an employer group health plan based on their own or a spouse’s current employment may be able to delay Part B, but coordination rules depend in part on the employer and the type of coverage involved.
The often-cited 20-employee rule is important because Medicare generally becomes the primary payer for workers age 65 or older when the employer has fewer than 20 employees, while the group health plan usually pays first when the employer has 20 or more employees. Other rules apply to people under 65 who qualify for Medicare because of disability and to those with end-stage renal disease, so employer size should not be treated as the only consideration. (medicare.gov)
Anyone working beyond 65 should confirm the rules directly with the employer’s benefits administrator before delaying Medicare. An employer plan can exist while still paying secondary to Medicare, which can create major coverage problems if someone failed to enroll when Medicare was supposed to be primary.
COBRA Is Not the Same as Active Employer Coverage
COBRA creates another potential trap because it allows people to continue an employer group health plan after employment ends. But Medicare does not generally treat COBRA as active-employment coverage for purposes of delaying Part B enrollment.
That means someone who leaves a job after 65 and simply continues COBRA can potentially miss the Part B Special Enrollment Period if they assume COBRA preserves the same Medicare rights. Medicare specifically advises beneficiaries to understand how COBRA interacts with Medicare before choosing to delay enrollment. (medicare.gov)
Prescription coverage requires its own analysis because COBRA drug coverage may still be creditable for Part D purposes. Part B and Part D therefore need to be evaluated separately rather than assuming one employer-benefit rule covers both.
Divorce Can Reduce IRMAA—but It Does Not Happen Automatically
Higher-income beneficiaries pay an Income-Related Monthly Adjustment Amount, or IRMAA, in addition to standard Part B and potentially Part D premiums. Social Security usually bases that surcharge on tax information from two years earlier, which means a major change in current income may not immediately appear in the normal calculation.
Divorce is one of the life-changing events that can support a request for a new IRMAA determination when it causes household income to decline. Beneficiaries can submit Form SSA-44 along with evidence of the life-changing event and updated or estimated income information. (ssa.gov)
Simply filing separately does not automatically produce a lower surcharge, however, and married-filing-separately rules can actually create particularly unfavorable IRMAA thresholds in some situations. Someone going through a divorce should evaluate the Social Security rules rather than assuming the premium will fall immediately after tax filing status changes.
Original Medicare and Medicare Advantage Solve Different Problems
It is tempting to frame Original Medicare with Medigap as the expensive option and Medicare Advantage as the inexpensive option. That comparison is too simplistic.
Medigap generally requires higher predictable premiums in exchange for reducing many of the medical bills that remain after Original Medicare pays. Medicare Advantage can offer much lower monthly premiums, but beneficiaries may incur copayments and coinsurance as services are used and may need to remain within a network or obtain prior authorization.
One approach shifts more cost toward premiums, while the other can leave more costs dependent on medical usage. The better fit depends on providers, travel patterns, prescriptions, health needs, budget and tolerance for unpredictable expenses rather than a universal rule about which plan is superior.
Don’t Choose a Plan Based on the Agent’s Incentive
Insurance agents can be valuable because Medicare is complicated, but beneficiaries should understand what products an agent is able to sell. Someone representing only a limited number of Medicare Advantage or Medigap carriers may not be comparing the full marketplace.
It is reasonable to ask whether the agent sells both Medicare Advantage and Medigap, which insurers they represent and how they are compensated. The recommendation should be based on the beneficiary’s needs rather than which product happens to produce the easiest enrollment or highest compensation.
Beneficiaries who want another perspective can also contact their State Health Insurance Assistance Program, commonly known as SHIP. These programs provide free Medicare counseling and can help explain enrollment rules without selling an insurance product.
Medical Savings Should Complement Coverage, Not Replace the Analysis
Some retirees prefer accepting more health-care risk and keeping the money they would otherwise spend on supplemental premiums in savings. That can be reasonable for someone with significant assets and a clear understanding of the potential exposure.
The danger is assuming a large savings account automatically makes Original Medicare’s unlimited Part B cost-sharing risk insignificant. A serious illness can create repeated expenses, while long-term savings also need to cover housing, inflation, travel and other retirement priorities.
The comparison should therefore be based on total expected costs and worst-case exposure. Paying more in premiums can feel wasteful during healthy years, while self-insuring can feel efficient until an expensive medical year arrives.
Medicare Enrollment Is Really Risk Management
Choosing Medicare coverage is not just about estimating next year’s doctor’s visits. It is about deciding how much financial uncertainty someone is willing to retain and how much they want to transfer to an insurer.
Original Medicare without supplemental coverage leaves substantial exposure because there is no annual out-of-pocket ceiling. Medigap can make those costs more predictable, while Medicare Advantage introduces its own annual spending limits but may also bring networks, prior authorization and different cost-sharing structures.
The cheapest option in a healthy year is not automatically the cheapest option over an entire retirement. A better Medicare decision considers the monthly premium, potential maximum exposure, doctors, prescriptions, ability to change coverage later and how much uncertainty the household can comfortably absorb.
Understanding those trade-offs before health changes is far easier than trying to solve them afterward.
