Medicare Part D: how drug coverage works and where the costs hide

Original Medicare doesn't cover most prescription drugs — that's the job of Part D, a separate, privately run plan you buy on top of Parts A and B (or that's bundled into a Medicare Advantage plan). Part D is where a lot of retirees quietly overpay, because the plans differ wildly on which drugs they cover, at what tier, and the 'best' plan depends entirely on the specific medications you take. The mechanics reward one habit above all: comparing plans every single fall.

The formulary decides your cost, not the drug

Each Part D plan has a formulary — its list of covered drugs, sorted into tiers from preferred generics (cheapest) up to specialty drugs (priciest). The same medication can be a $5 copay on one plan and a $90 copay (or not covered at all) on another. Plans also use tools like prior authorization, step therapy, and quantity limits. This is why two retirees on identical incomes can pay hundreds of dollars a year apart for the same prescriptions — the plan, not the pharmacy, is the variable.

The coverage phases (and the cap that changed everything)

Part D spending moves through phases each year: a deductible (if the plan has one), then an initial coverage phase where you pay copays, and historically a 'donut hole' coverage gap that exposed people to high costs. The major recent change: federal law now caps annual out-of-pocket Part D drug spending — around $2,000 (indexed) — after which your covered drugs cost $0 for the rest of the year. That cap, plus the option to spread payments monthly, has transformed the math for people on expensive medications.

How to actually pick a plan

1. 1
   List your exact drugs and doses

   Plan cost is meaningless in the abstract. Enter every medication, dose, and preferred pharmacy into Medicare.gov's Plan Finder — it prices each available plan against your real prescription list.

2. 2
   Compare total annual cost, not premium

   The Plan Finder shows estimated yearly cost including premium, deductible, and copays. A $0-premium plan can be the most expensive overall if it tiers your drugs poorly.

3. 3
   Check restrictions on your drugs

   Look for prior authorization, step therapy, and quantity limits on the medications you actually take — these can turn a cheap-looking plan into a hassle or a denial.

4. 4
   Re-shop every fall

   Plans change their formularies, tiers, and premiums every January, and your prescriptions change too. Re-run the comparison during Medicare open enrollment (Oct 15–Dec 7). Most beneficiaries never do — and overpay for it.

The bottom line

Part D is the drug-coverage layer where the plan you pick — matched to the exact medications you take — decides your cost far more than the pharmacy does. Enroll on time to dodge the lifelong penalty, take advantage of the new annual out-of-pocket cap and monthly payment option, and above all re-run the Plan Finder every fall against your current prescriptions. Twenty minutes of comparison shopping is routinely worth hundreds of dollars a year, and it's the chore four out of five beneficiaries skip.