Lifestyle

5 Medicare Enrollment Mistakes Most Seniors Make Between Now and December

By Mike Harper · September 1, 2026

Open enrollment starts October 15. Most people will do what they did last year — keep the same plan without checking whether it still covers their medications at the same cost. That default decision costs millions of seniors money every year.

Medicare open enrollment runs from October 15 through December 7. It’s the one window when you can switch Medicare Advantage plans, change Part D drug plans, or move between Original Medicare and Advantage. What you choose — or don’t choose — locks in your coverage, your costs, and your provider access for all of 2027.

Keeping your current plan without reviewing the Annual Notice of Change. Every Medicare plan is required to mail you an Annual Notice of Change by September 30. This document details everything that’s changing for the coming year — formulary changes, premium adjustments, provider network modifications, and benefit alterations. Most people throw it away. The drug that was Tier 2 this year may be Tier 4 next year — meaning a copay that triples without you knowing until January.

Choosing a plan based on the monthly premium alone. The lowest-premium plan often has the highest out-of-pocket costs. A $0 premium Medicare Advantage plan with a $8,000 out-of-pocket maximum and narrow provider network can cost far more in a year with significant medical needs than a $45 premium plan with a $4,000 maximum and broader access. Total cost — premium plus deductible plus copays plus coinsurance — is the number that matters.

Not checking whether your doctors are still in-network. Medicare Advantage networks change annually. Your cardiologist, your primary care doctor, and your specialist may all have been in-network last year and out-of-network next year. The plan’s provider directory — not the one from last year, but the one for the coming year — is the only way to verify. If your doctors are out of network, every visit is either uncovered or billed at out-of-network rates.

Not running your specific prescriptions through the Medicare Plan Finder. The Medicare.gov Plan Finder tool lets you enter your exact medications, dosages, and preferred pharmacy, then compares every available plan’s total annual drug cost — premiums, deductibles, copays, and coverage gap costs — specific to your prescription list. Most seniors don’t use it. Running the comparison takes 15 minutes and can reveal differences of $1,000 or more between plans.

Missing the December 7 deadline and getting locked in. If you don’t make a change by December 7, you’re enrolled in whatever plan you had — with whatever changes the Annual Notice described. There is no grace period. There is no January do-over for most beneficiaries. The Medicare Advantage Open Enrollment Period in January through March allows Advantage enrollees to switch to a different Advantage plan or return to Original Medicare — but it doesn’t allow Original Medicare enrollees to join an Advantage plan. The December 7 deadline is the one that matters most for the largest number of people.

The default is inertia. The system is designed around the assumption that most people won’t check, won’t compare, and won’t switch. Every year, that assumption saves insurance companies money and costs beneficiaries more than they needed to pay.