- Auto-renewing without reading your Annual Notice of Change is the #1 expensive mistake.
- Always check YOUR drugs against the formulary — tiers and restrictions change yearly.
- Compare total annual cost, not just the monthly premium.
Open enrollment (October 15 – December 7) is your annual chance to get prescription drug coverage right. It's also when the most expensive mistakes happen — usually from rushing or from assuming "my plan is fine." Here are the five costliest errors and how to dodge each one.
Mistake 1: Auto-renewing without reading the Annual Notice of Change
Your plan must send an Annual Notice of Change (ANOC) by late September. Premiums, deductibles, copays, formularies, and pharmacy networks all change year to year. The plan that was perfect in 2026 can quietly become expensive in 2027. Fix: read the ANOC the week it arrives and compare at least two alternatives.
Mistake 2: Shopping on premium alone
A $0-premium plan feels like a win — until your $350/month brand drug lands on a non-preferred tier with 40% coinsurance and prior authorization. Fix: do full-year math: (premium × 12) + deductible + your drugs' copays. The "cheap" plan often loses.
Mistake 3: Not checking YOUR drugs in the formulary
Formularies change every year. A drug that was Tier 2 (preferred brand) can move to Tier 3, gain a prior-authorization requirement, or drop off entirely. Fix: look up every prescription — name, dosage, frequency — in each plan's upcoming-year formulary before you enroll.
Mistake 4: Ignoring the pharmacy network
Plans use preferred vs. standard pharmacies, and the same drug can cost double at a standard pharmacy. Networks change annually, and mail-order terms change too. Fix: confirm your pharmacy is still preferred, and price your maintenance drugs through the plan's mail-order option.
Mistake 5: Missing the December 7 deadline
December 7 is a hard cutoff. Miss it and you're generally locked into your current plan until next fall — even if your drug was dropped from the formulary. (Limited exceptions exist via Special Enrollment Periods.) Fix: finish your comparison by early December; don't start on December 6.
Bonus: forgetting Extra Help
If your income and resources are limited, Extra Help (the Low-Income Subsidy) can cover premiums, deductibles, and most copays — worth thousands per year. Many eligible people never apply. Check eligibility at ssa.gov/extrahelp; it costs nothing to apply.
Bottom line: Fifteen focused minutes — your drug list, your ANOC, and two plan comparisons on total annual cost — is all it takes to avoid the mistakes that cost people hundreds every year.