Quick answer: The most common — and costly — Medicare mistakes are: missing your Initial Enrollment Period, never reviewing your plan during the Annual Enrollment Period (Oct 15–Dec 7), choosing the wrong plan type for your health needs, ignoring your plan's drug formulary, and picking a Medicare Advantage plan that doesn't include your doctors. Each one is avoidable with a yearly review of your coverage.

Why These Mistakes Matter

Some Medicare decisions are hard to undo. Late enrollment penalties can last as long as you have the coverage, and a plan that doesn't match your doctors or prescriptions can cost more than you expected. Understanding the most common mistakes helps you avoid them and make informed decisions about your healthcare coverage.

As an independent Medicare broker, I've seen these mistakes repeatedly. The good news is that all of them are preventable with the right information and guidance. Let's walk through the five most critical mistakes I see beneficiaries make and how to avoid them.

Mistake #1: Missing Your Initial Enrollment Period Deadline

Why This Matters

Your Initial Enrollment Period (IEP) is the seven-month window centered around your 65th birthday — the three months before your birthday month, your birthday month, and the three months after — when you can enroll in Medicare Part B without penalties. If you miss this window and don't have coverage based on current employment, a late enrollment penalty is added to your Part B premium.

The Part B late enrollment penalty is an extra 10% for each full 12-month period you could have had Part B but didn't sign up. If you delay three years, you pay 30% more. On the 2026 standard premium of $202.90 per month, that is about $61 more each month, and in most cases you pay it for as long as you have Part B.

Because the penalty is a percentage of the standard premium, the dollar amount rises whenever the premium rises. At 2026 rates, a 30% penalty comes to about $730 a year.

The Same Applies to Part D

Prescription drug coverage has similar penalties. If you go 63 days or more in a row without Part D or other creditable drug coverage after you are first eligible, you pay an extra 1% of the national base beneficiary premium ($38.99 in 2026) for each full month you went without coverage. The penalty is added to your Part D premium for as long as you have Part D.

How a Broker Can Help

A broker can walk you through your IEP timeline, help you check whether your employer coverage lets you delay Part B (if it does, you get a Special Enrollment Period when that coverage ends), and remind you of the dates that apply to you.

Key Takeaway

Enroll during your IEP unless you have coverage based on current employment that lets you delay. If the Part B premium is hard to afford, ask about a Medicare Savings Program. In California you apply through your county Medi-Cal office, and for people who qualify the program pays the Part B premium.

Mistake #2: Not Reviewing Your Medicare Plan Each Year During Annual Enrollment

Why Plans Change Every Year

Medicare plans are NOT static. Every October 15 through December 7, there's an Annual Enrollment Period when you can change plans. During this window, plan premiums change, formularies (drug lists) change, provider networks change, and coverage benefits change.

The plan that was a good fit in 2026 might be a poor fit in 2027. A medication you take might be moved to a higher cost tier, or a doctor you see might be dropped from the network. Your out-of-pocket costs could increase by hundreds of dollars, but you'll never know if you don't review your options.

Real Example of This Mistake

Karen enrolled in Medicare Advantage Plan A in 2024 and hasn't looked at her options since. In 2026, her primary care doctor left the plan's network, but she doesn't realize it until she tries to schedule an appointment. She could have switched during the Annual Enrollment Period to a plan that includes her doctor. Because she is in a Medicare Advantage plan she has one more chance: the Medicare Advantage Open Enrollment Period, January 1 to March 31, allows one change. After March 31 she would generally have to wait for the next Annual Enrollment Period unless she qualifies for a Special Enrollment Period.

How a Broker Can Help

Many brokers contact clients before the Annual Enrollment Period each year, review the current plan for changes, check drug formularies and doctor networks, and compare the plans they represent against the client's situation. Many brokers set calendar reminders and proactively reach out so clients don't miss the enrollment window.

Key Takeaway

Review your coverage annually even if you were satisfied the previous year. Set aside an hour or two during the Annual Enrollment Period to review your options. It can help you avoid surprise costs, coverage gaps and network disruptions.

Mistake #3: Choosing the Wrong Plan Type for Your Health Situation

Medicare Advantage vs. Medicare Supplement Mismatch

Medicare Advantage and Medigap handle costs in opposite ways, and people sometimes pick one without looking at the other. Some choose a plan for its low premium and are surprised by copays when they need a lot of care. Others pay a higher premium for flexibility they don't end up using.

The mistake stems from not understanding the fundamental differences: Medicare Advantage offers low premiums but requires network use and has per-visit copayments; Medigap offers comprehensive coverage but with higher premiums. Either mismatch can cost more than expected.

Example of This Mistake

David is 67 and healthy. He chooses a plan type by looking at one number, the monthly premium, without checking how each type handles his doctors, his prescriptions, or a year when he needs a lot of care. Two things are easy to miss. A lower premium usually means paying more as you use care, and a higher premium usually means paying less. And in California the order can matter: if you start in Medicare Advantage and later want a Medigap policy, the insurer can generally ask health questions unless a guaranteed-issue right applies. Our guide to switching from Medicare Advantage to Medigap in California explains those rights.

How a Broker Can Help

A broker analyzes your health situation, current medications, doctors, and anticipated healthcare needs. They run cost estimates under both plan types, show you real numbers for your specific situation, and make personalized recommendations. They help your plan choice line up with your actual healthcare usage, not generic guidance.

Key Takeaway

There's no universally "best" Medicare plan. The right choice depends entirely on your individual situation. Get analysis of your specific scenario from an independent broker before choosing.

Mistake #4: Not Verifying Drug Formularies and Switching Medications Unnecessarily

The Problem

Medicare Part D drug lists (formularies) can have hundreds of medications. Not all plans cover all medications, and covered medications may have different copayment amounts depending on their tier. Some beneficiaries choose plans without checking whether their medications are covered, only to discover afterward that they must pay full price for medications or switch to alternatives.

Even worse, some beneficiaries accept drug switches suggested by their doctors due to formulary restrictions, only to later discover they could have chosen a different Medicare plan that covered their original medication.

Example of This Mistake

James takes a brand-name blood pressure medication that costs $200/month. He enrolls in Medicare Advantage Plan X without checking its formulary. After enrollment, he discovers the plan doesn't cover his medication or covers it only at a high tier requiring $75/month copayment ($900/year additional cost). His doctor suggests switching to a generic alternative that the plan does cover well. A different Medicare Advantage plan would have covered his original medication with a $10 copayment. James should have checked formularies first before enrolling.

How a Broker Can Help

A Medicare broker requests a list of all your current medications and reviews the formularies of the plans they represent before recommending one. They identify any medications that aren't covered or are covered at high cost tiers. If your medication isn't well-covered, they recommend alternative plans. The aim is to keep you on your current medications at a cost you expected.

Key Takeaway

Before enrolling in any plan, verify that all your medications are covered. Review the copayment amount for each medication under the specific plan. A plan with a low premium but expensive drug coverage might cost more overall than a plan with higher premium but comprehensive drug coverage.

Mistake #5: Choosing Medicare Advantage Plans with Out-of-Network Doctors

The Network Issue

Medicare Advantage plans restrict you to in-network providers. If you see doctors outside the network, you pay significantly higher costs or the services aren't covered at all (with HMO plans). Many beneficiaries choose plans without confirming that their preferred doctors participate in the network.

This becomes a real problem when someone switches to a Medicare Advantage plan, then later learns their long-time primary care doctor or important specialist isn't in the network. They must either change doctors or face high out-of-pocket costs.

Example of This Mistake

Margaret has seen the same cardiologist for 15 years. She enrolls in Medicare Advantage Plan Y with a $0 premium because it looks appealing. After enrollment, she discovers her cardiologist doesn't participate in the plan's network. She has two bad options: find a new cardiologist (disrupting her long-term care relationship) or stay with her current cardiologist but pay out-of-network costs (potentially $300+ per visit). She should have verified her doctor's participation before enrolling.

How a Broker Can Help

Before recommending a Medicare Advantage plan, a broker verifies that your current primary care doctor and key specialists are in-network. They check the plan's provider directory or contact the plan directly to confirm. If your preferred doctors aren't in the plan, they recommend alternatives that do include them. This helps with continuity of care and can help you avoid forced doctor switches.

Key Takeaway

If you choose Medicare Advantage, verify that all your current doctors participate in the plan's network BEFORE enrolling. Don't assume your doctor is in the network—verify it directly with the plan or your broker.

Bonus Mistake: Not Checking for IRMAA (Income-Related Monthly Adjustment Amounts)

The Hidden Cost

If your income exceeds specific thresholds, you'll pay additional surcharges (IRMAA) on top of your regular Medicare premiums. These surcharges apply to Part B and to Part D, including the drug coverage in a Medicare Advantage plan. Many beneficiaries don't know about IRMAA until they receive their bills and discover they're paying more than expected.

IRMAA is calculated based on your Modified Adjusted Gross Income (MAGI) from two years prior, so retirement income, investment gains, and withdrawals from retirement accounts can trigger surcharges. Some beneficiaries can reduce IRMAA by requesting an appeal if their income decreased due to major life changes (retirement, loss of income, etc.).

How a Broker Helps

A broker understands IRMAA thresholds and asks about your income situation during your initial consultation. They help you understand whether IRMAA applies to you and how to estimate your costs. If your income is close to IRMAA thresholds, they discuss strategies to manage it (such as timing of Roth conversions or charitable contributions) and explain the process for requesting appeals if your income changes. Talk to a tax professional before making income-timing decisions.

How to Prevent These Mistakes: Work with an Independent Medicare Broker

What Brokers Do

An independent Medicare broker represents multiple insurance companies and is paid by those companies, meaning services are free to you. Brokers spend time understanding your individual situation, analyzing plan options, and making personalized recommendations. They handle enrollment paperwork, follow up to confirm coverage is active, and provide ongoing support when issues arise.

Questions to Ask a Broker

Important: Ask any agent how many companies they represent. An agent who represents one company can only show you that company's plans. Be cautious with anyone who pressures you toward a plan without asking about your doctors and prescriptions.

Key Takeaways

Moving Forward

Medicare has a lot of moving parts, and some choices are hard to change later. You don't have to sort it out alone. An independent broker can compare the plans they represent against your doctors and prescriptions, and free, unbiased counseling is available from California's HICAP at 1-800-434-0222.

If you'd like a second set of eyes on your situation, reach out and we can go through it together.

Sources & further reading

Last reviewed: October 2026. Enrollment dates and figures reflect 2026 and can change each year — always confirm current details at Medicare.gov. This article is educational and not individualized advice.