How Does Medicare Part D Work, and What Is the Out-of-Pocket Cap?

Original Medicare (Parts A and B) does not cover most retail prescriptions. Part D does, and it is delivered by private insurance companies that contract with Medicare. You get it one of two ways: a stand-alone prescription drug plan that sits alongside Original Medicare, or a Medicare Advantage plan that includes drug coverage. Each plan sets its own premium, formulary (its list of covered drugs), tiers, and pharmacy network, within rules set by the Centers for Medicare & Medicaid Services (CMS). That is why two people in the same Arizona county can pay very different amounts for the same prescription.

The 2026 standard benefit has three stages, because the coverage gap, often called the donut hole, was eliminated starting in 2025 under the Inflation Reduction Act. First is the deductible, which can be up to $615 in 2026. A plan may charge less or none, so check the plan, not the maximum. Second, after the deductible, you pay 25% of the cost of covered drugs, though many plans charge flat copays by tier instead, so confirm how yours works. Third, once your out-of-pocket costs for covered drugs reach $2,100 in 2026, you pay $0 for covered drugs for the rest of the calendar year. The 2025 cap was $2,000, so any source citing that figure is describing the prior year. For 2027, CMS set the out-of-pocket threshold at $2,400 and the maximum standard deductible at $700.

What counts toward the $2,100 matters. Generally, the amounts you pay for covered drugs, including the deductible, coinsurance, and copays, count toward the cap. Check your plan's materials for exactly what counts, including whether premiums and drugs your plan does not cover are excluded, since details vary by plan and drug.

Enrollment timing carries a lasting cost. If you go 63 or more days in a row after your Initial Enrollment Period without Part D or other creditable drug coverage, Medicare adds a late enrollment penalty to your Part D premium. The penalty is 1% of the national base beneficiary premium for each full uncovered month, rounded to the nearest 10 cents, and it is added to your monthly Part D premium. Medicare.gov explains the penalty on its Part D late enrollment penalty page. The 2026 national base beneficiary premium is $38.99 and the 2027 figure is $41.33. CMS resets it each year, so the dollar penalty can change annually. Drug coverage from a current employer can count as creditable coverage, so ask the plan sponsor for a written notice before you assume you are covered.

Three other pieces round out the picture. The Medicare Prescription Payment Plan is an opt-in option that spreads your out-of-pocket drug costs across the calendar year in monthly bills instead of charging them at the pharmacy counter. It does not lower what you owe. Extra Help, also called the Part D Low-Income Subsidy, lowers premiums and drug costs for people under income and resource limits that the Social Security Administration (SSA) sets and updates, and the resource test counts most savings and investments, so confirm current limits with SSA. Higher-income enrollees pay an extra income-related amount on top of the Part D premium, and the IRMAA articles linked below explain the income lookback and the brackets.

The cap changes the risk profile more than it changes the average bill. Before 2025, a bad year of specialty drugs had no ceiling for the person paying. In 2026 there is a ceiling on covered drugs, so the question for a household with a $1 million to $3 million portfolio is less about catastrophic exposure and more about premiums, pharmacy access, formulary changes, and cash flow timing. In an illustrative case, $12,000 of gross covered drug costs in 2026 would reach the $2,100 cap. The cap is reached at $6,555 of gross covered drug costs: the $615 deductible, plus 25% of the remaining $5,940, which is $1,485, for $2,100 total. After that, additional covered drug costs add $0 for the rest of that year.

Timing within the year is where the Medicare Prescription Payment Plan can matter. In an illustrative case with one covered drug at $1,000 in gross cost each month under the standard 2026 benefit, the pharmacy would charge $711.25 in January, $250.00 a month from February through June, $138.75 in July, and $0 from August through December, for $2,100 total. The payment plan smooths that out. For someone who has paid nothing toward the cap yet and joins in January, the first month's bill is capped at the lesser of the 2026 cap divided by 12, which is $175.00, or the actual cost. Any amount already paid at the pharmacy before joining lowers that figure. Your plan calculates its own bills from there, so ask it how the later months work. The tradeoff is smaller payments early in the year in exchange for larger ones later.

Open enrollment, October 15 to December 7, is the time to check the things that move every year. Read your Annual Notice of Change from your plan, then run your actual prescriptions and your actual pharmacy through the Medicare Plan Finder on Medicare.gov, and confirm next year's deductible, drug tiers, and cap in the plan's own documents. If you want free, independent help with that, Arizona's State Health Insurance Assistance Program (SHIP) offers counseling that is not tied to an insurer. Search for Arizona SHIP on the Arizona Department of Economic Security website to find current contact details. For anyone planning Roth conversions or large IRA withdrawals, the income you report can affect your Medicare premiums in later years. Singh PWM can model that income side of the picture, but plan selection and enrollment decisions run through Medicare, your plan, and SHIP.

• Letting a plan auto-renew without reading the Annual Notice of Change. Formularies, tiers, deductibles, and pharmacy networks can all change on January 1.
• Comparing plans by premium alone. A low-premium plan can cost more in total if your drugs are on a higher tier, need prior authorization, or are excluded.
• Assuming the $2,100 cap in 2026 includes premiums. It does not, and it covers only drugs your plan covers.
• Dropping or skipping Part D with no creditable coverage because you take no medications today. A 63-day gap after your Initial Enrollment Period can create a late enrollment penalty added to your Part D premium.
• Quoting last year's numbers. The 2025 cap of $2,000, the 2026 cap of $2,100, and the 2027 threshold of $2,400 are all different, and your plan's terms must come from its own notice or Medicare.gov, not from an old article.