If you have Original Medicare (Part A and Part B), you have probably noticed it does not pay for everything. You are still responsible for deductibles, copayments, and coinsurance, and unlike most employer plans, Original Medicare has no annual cap on what you can spend out of pocket. Medigap, also called Medicare Supplement Insurance, is designed to fill those gaps. This guide explains how it works, what it costs, when to enroll, and the special rules that apply here in Illinois.
What is Medigap?
Medigap is private health insurance you buy in addition to Original Medicare. It supplements your Medicare coverage by paying some or all of the costs Medicare leaves for you, such as the Part A hospital deductible, Part B coinsurance, and the daily costs of a long hospital stay. Because it works alongside Original Medicare rather than replacing it, Medicare pays its share of a covered claim first, and then your Medigap policy pays its share automatically.
A few things make Medigap distinct from other Medicare coverage:
- No networks. You can use any doctor, hospital, or provider in the country that accepts Medicare. There are no HMO or PPO networks and no referrals.
- Standardized benefits. Plans are labeled with letters (A, B, C, D, F, G, K, L, M, and N). A given letter covers exactly the same benefits no matter which company sells it, so a Plan G from one insurer is identical in coverage to a Plan G from another.
- Guaranteed renewable. As long as you pay your premium, your policy cannot be canceled because of health problems.
- One person per policy. Unlike employer coverage, a Medigap policy covers only you. Your spouse would need their own.
What does Medigap cover?
Every standardized Medigap plan helps with Medicare’s cost-sharing, but the letters differ in how much they cover. The chart below shows the most common plans available to people newly eligible for Medicare today.
| Benefit | Plan G | Plan N | Plan K |
|---|---|---|---|
| Part A coinsurance & hospital costs | 100% | 100% | 100% |
| Part B coinsurance/copayment | 100% | 100%* | 50% |
| Blood (first 3 pints) | 100% | 100% | 50% |
| Part A hospice coinsurance | 100% | 100% | 50% |
| Skilled nursing facility coinsurance | 100% | 100% | 50% |
| Part A deductible | 100% | 100% | 50% |
| Part B deductible | Not covered | Not covered | Not covered |
| Foreign travel emergency | 80% | 80% | Not covered |
*Plan N pays the Part B coinsurance in full except for copays of up to $20 for some office visits and up to $50 for emergency-room visits that do not lead to an inpatient admission. Benefit details are set by federal standardization; plan availability varies by carrier and area.
A note on Plans C and F
Plans C and F, which also paid the Part B deductible, are no longer available to people who became eligible for Medicare on or after January 1, 2020. If you were eligible before that date, you can generally still buy or keep them. For most people new to Medicare, Plan G is now the most comprehensive option and the most popular choice, because it covers everything except the annual Part B deductible ($283 in 2026).
High-deductible options
There is also a High-Deductible Plan G (and High-Deductible Plan F for those still eligible). These charge a much lower monthly premium, but you pay Medicare-covered costs yourself until you meet an annual deductible, $2,950 in 2026, after which the plan pays like a standard Plan G.
What Medigap does not cover
Medigap does not include prescription drug coverage. If you want help paying for medications, you will enroll in a separate stand-alone Medicare Part D drug plan. Medigap also does not cover routine dental, vision, hearing aids, or long-term custodial care. These “extras” are sometimes bundled into Medicare Advantage plans instead, as explained below.
Medigap vs. Medicare Advantage
People often weigh Medigap against Medicare Advantage, and they work very differently. Medigap keeps you on Original Medicare and adds predictable, nationwide coverage with few surprises, but it carries a monthly premium and does not include drugs or extra benefits. Medicare Advantage replaces Original Medicare with a private plan that often has a low or $0 premium and bundles in drug, dental, or vision benefits, but it uses provider networks and requires you to manage copays up to an annual out-of-pocket maximum. Neither is “better” for everyone; the right fit depends on your budget, your doctors, and how you like to manage care.
How much does Medigap cost?
Because benefits are standardized, the biggest differences between companies come down to price and service. Premiums for the same plan letter can vary widely, and insurers use one of three pricing methods:
- Community-rated: everyone pays the same premium regardless of age.
- Issue-age-rated: your premium is based on your age when you buy, and does not rise simply because you get older.
- Attained-age-rated: your premium starts lower but increases as you age.
All three can still rise over time due to inflation and other factors, so a lower starting premium is not always the cheapest plan over the long run. Comparing identical plan letters across carriers is the simplest way to make sure you are getting the best value.
When can you enroll in Medigap?
The best time to buy is during your Medigap Open Enrollment Period, a one-time, six-month window that starts the first month you are both age 65 or older and enrolled in Medicare Part B. During this window you have a “guaranteed issue” right: an insurer must sell you any Medigap policy it offers, cannot charge you more because of your health, and cannot make you wait for coverage of pre-existing conditions.
Once that six-month window closes, insurers in most situations can use medical underwriting, reviewing your health history and potentially charging more or declining coverage, unless you qualify for a special guaranteed-issue right. That is why timing matters so much with Medigap.
Illinois Medigap rules you should know
Illinois offers some extra protections beyond the federal rules:
- 45-day birthday rule (effective May 2025). Each year, Illinois beneficiaries aged 65 to 75 get a 45-day window beginning on their birthday to switch to a Medigap plan with equal or lesser benefits without medical underwriting. The switch is generally limited to your current insurer or its affiliated companies, and this opportunity ends at age 75.
- Protection when leaving Medicare Advantage. Under Illinois House Bill 2775, insurers cannot deny coverage or charge more for pre-existing conditions when you move from a Medicare Advantage plan back to Original Medicare within 30 days of the new plan’s effective date.
- Under-65 access. Illinois requires that Medigap plans be guaranteed issue for eligible beneficiaries under 65 (for example, those on Medicare due to disability), with premiums capped at the rates charged to those over 65.
State rules change and can have specific conditions; confirm your situation with a licensed agent before making a switch.
Frequently asked questions
Do I need Medigap if I already have Medicare Advantage?
No, and you generally cannot use both. Medigap only works with Original Medicare. If you are enrolled in a Medicare Advantage plan, a Medigap policy cannot pay toward your Advantage costs.
Does Medigap cover my prescriptions?
No. You will need a separate Medicare Part D drug plan for medication coverage.
Can I switch Medigap plans later?
Yes, but outside a guaranteed-issue window (like the Illinois birthday rule) you may have to answer health questions and could be charged more or turned down. It is worth reviewing your options with an agent each year.
Is Plan G or Plan N better?
Plan G offers the most complete coverage aside from the Part B deductible. Plan N usually has a lower premium in exchange for small office and ER copays. The better choice depends on how often you see doctors and how you feel about occasional copays versus a slightly higher premium.
Talk to a licensed Illinois Medicare agent
Choosing the right Medigap plan, and timing your enrollment to avoid underwriting, is easier with someone who does it every day. Illinois Health Agents can compare identical plan letters across carriers to find your best rate, at no cost to you.