If you’re shopping for a Medicare Part D plan, it probably seems straightforward. You sign up for a plan that covers your prescriptions, pay a monthly premium, and head to the pharmacy when you need something. Simple enough, right?
Not really.
Every fall, I review countless drug plans for my clients. And every year, I see people stuck with plans that no longer fit, costs that keep climbing, and confusion about why their coverage doesn’t look the way they expected. Choosing the wrong plan can be a costly mistake—and it happens more often than people think.
If you’re about to choose a Part D plan—or already have one—here are the biggest issues to watch out for.
1. Drug Lists (Formularies) Vary by Plan
Just because you have drug coverage doesn’t mean your specific medications are included. Each insurance company creates its own formulary—a list of drugs they agree to cover—and they decide which drugs go on which tiers, and which get left out entirely.
One plan might have your cholesterol medication as a Tier 1 generic with a $0 copay. Another plan might list the same drug as Tier 3 with a $45 copay—or not cover it at all.
This isn’t just about obscure medications. I’ve seen common drugs like Ventolin, Xarelto, or Eliquis show up differently across multiple plans. That’s why I never recommend picking a plan based on premium alone. The real cost is what you pay at the pharmacy.
2. Tiered Pricing Makes Costs Hard to Predict
Most people assume their copay is fixed. But in Medicare drug plans, pricing depends on drug tiers, which typically range from Tier 1 (generics) to Tier 5 or 6 (specialty medications). Your tier determines your cost.
A drug that’s Tier 2 on one plan might be Tier 3 on another, which could mean the difference between a $5 copay and a $60 copay. On top of that, your pharmacy matters too. Some plans give lower prices at preferred pharmacies only. And guess what? Preferred status can change every year.
I’ve had clients go to the same pharmacy they’ve always used, only to find out their plan no longer gives preferred pricing there. Their $9 medication jumps to $42 overnight—and they don’t find out until checkout.
3. Step Therapy and Prior Authorization Still Exist
Even if your drug is on the formulary, it doesn’t mean you can just walk in and get it filled. Many plans require prior authorization before they’ll cover certain medications. That means your doctor has to submit paperwork to explain why you need that specific drug.
Worse, some plans impose step therapy—a rule that says you must try a cheaper drug first, even if you and your doctor already know it doesn’t work for you.
These hurdles are frustrating. They cause delays, create stress, and put the insurance company between you and your doctor’s medical judgment. I see it all the time with insulin, inhalers, anti-depressants, and pain medications.
4. Plans Change Every Year, Even if You Don’t
This is a big one. Your drug plan is only guaranteed to stay the same for one year. Every fall, insurance companies can (and often do) make changes to:
- The drugs they cover
- The tier placement of each drug
- Which pharmacies they consider preferred
- Monthly premiums and copays
So even if your plan worked great this year, it might not next year.
I’ve seen plans drop medications entirely or shift drugs up to a higher tier. A $0 generic becomes a $25 brand-name overnight. If you’re not checking every fall—and most people don’t—you could end up overpaying or losing access to a drug you depend on.
5. Many People Pick the Wrong Plan
Part D plans aren’t one-size-fits-all. The best plan for one person might be terrible for someone else. Yet most people pick a plan based on just two things: the monthly premium and the name of the insurance company.
That’s a mistake.
The lowest premium plan might cost you more in total if it doesn’t cover your medications well or if it charges high copays at your pharmacy. I’ve seen clients switch from a $7/month plan to a $20/month plan and save hundreds because the new plan covered their drugs more efficiently.
When I review plans for clients, I look at total annual cost: premium + copays + pharmacy match. That’s the only number that matters.
6. You’re Locked In After Enrollment
Once you enroll in a Part D plan, you’re usually committed to it for the calendar year. Unless you qualify for a Special Enrollment Period—like moving or losing other coverage—you can’t switch until the next Annual Enrollment Period in the fall.
That means if you pick a plan and realize in February that your main medication isn’t covered the way you thought, you could be stuck with that mistake until the following January.
That’s why it’s so important to get it right the first time. And why I always recommend a personalized review instead of guessing or defaulting to a low premium.
7. Out-of-Pocket Costs Can Still Be High—Even with a Cap
There is now a maximum amount you’ll pay out-of-pocket in a year for your medications, which is a huge improvement from how Part D used to work. But that doesn’t mean costs are low.
You still pay 100% of your actual costs—including deductible, copays, and coinsurance—until you hit that annual limit. If your drugs are expensive, you can reach that cap quickly. And even generic drugs can be surprisingly expensive if they’re placed in a higher tier.
Some medications, like certain cancer treatments or brand-name inhalers, can cost hundreds per refill—even with a Part D plan.
8. Sometimes You’re Better Off Not Using Your Plan
This one surprises people the most. Sometimes it’s cheaper to pay cash for your prescription instead of using your insurance.
Services like GoodRx, SingleCare, Mark Cuban’s Cost Plus Drugs, and even some local pharmacy cash discounts can beat your Part D copay—especially on common generics. Your pharmacist may or may not tell you this up front, and your insurance company definitely won’t.
It’s completely legal and perfectly smart to skip your insurance and pay the lower price out of pocket. You just need to know that those cash purchases don’t count toward your drug plan’s out-of-pocket max.
Still, for many people, the savings are worth it. I always tell clients: don’t feel weird about going around your plan. Use whatever method gives you the best deal. It doesn’t mean you picked a bad plan—it just means you’re using the system in the smartest way possible.
Final Thoughts
Part D plans are more complicated than they look. The idea of “just get a drug plan” turns into this maze of formularies, tiers, authorizations, and changing coverage. And the sad part is, most people don’t even realize they made a mistake until they’re at the pharmacy counter getting hit with a huge bill.
I help clients avoid those traps by comparing all the options—not just based on premium, but on your actual medications, your preferred pharmacy, and your total yearly cost. If you want help sorting it out, reach out. I’ll make sure you get the plan that works best for you, not just the one that looks cheap on the surface.