Medicare Prescription Payment Plan: how each monthly bill is calculated

The Medicare Prescription Payment Plan is an optional way to pay Part D drug costs that every Medicare drug plan, including Medicare Advantage plans with drug coverage, has had to offer since January 1, 2025. Medicare.gov describes it in one sentence: you get a bill from your plan instead of paying the pharmacy. The interesting part is how that bill is sized, because the formula is written into the Social Security Act and produces bills that can rise, fall or stay flat depending on when your costs arrive.

Labeled, hypothetical numbers below let you predict your own bills. Money is the only subject: whether a drug is right for you belongs with your prescriber, and which plan to choose is yours.

The formula, in two lines

Section 1860D-2(b)(2)(E)(iv) of the Social Security Act sets two different calculations, and CMS's final part one guidance from February 29, 2024 explains how plans must apply them.

For 2027 the out-of-pocket threshold is $2,400, set by CMS in its CY2027 Rate Announcement. Our separate guide to the Part D out-of-pocket cap explains what counts toward that threshold. Here the threshold matters for one reason only: it is the starting number in the first-month formula.

Three consequences follow directly from the two lines. Your first bill is never more than the cap divided by the months left. Months with no new prescriptions still produce a bill if you owe a balance, because the balance is spread forward. And the bills always add up to exactly your out-of-pocket costs for the year, with December's amount billed and paid early in the following year, as the CMS guidance notes.

Three sign-up scenarios, worked month by month

The inputs below are invented for the arithmetic and are not prices for any drug. Each scenario uses the 2027 threshold of $2,400. Amounts are rounded to cents, and in real bills a plan's rounding can shift a cent or two between months.

Scenario A: joined during open enrollment, large cost in January

Hypothetical inputs: you opted in before January 1, and your January fills alone bring your out-of-pocket costs to the full $2,400. You have no further out-of-pocket costs that year, because you reached the cap.

Twelve bills of $200 equal the $2,400 paid. Without the program, the same person would have paid $2,400 at the pharmacy in January. This is the case the program was built for.

Scenario B: joined in July, large cost starting in July

Hypothetical inputs: no Part D costs from January to June, then fills in July that reach the $2,400 cap. You opt in during July.

The same $2,400, but spread over six bills of $400 instead of twelve of $200. A later start halves the number of months and doubles the size of each bill. That is not a penalty; it is the formula running out of calendar.

Scenario C: joined in July after paying the pharmacy in the spring

Hypothetical inputs: you paid $1,000 at the pharmacy between January and June without the program, then opt in during July, when new fills cost you the remaining $1,400 to reach the cap.

Here the first-month formula subtracts what you already paid toward the threshold. The $1,000 paid in the spring stays paid; the program only spreads the $1,400 that remains.

When costs arrive in pieces, the bill moves

The three scenarios above put all the cost in one month, which makes the bills flat. Real fills rarely line up that neatly, and the subsequent-month formula reacts to every new cost. A short, hypothetical run shows the effect: you join before January, then incur $600 in January, nothing in February, and $600 in March.

The February bill drops sharply because there were no new costs to add, and March rises again when a new fill arrives. Neither month is a mistake. If you budget for a fixed monthly payment, this is the pattern to expect when prescriptions are filled irregularly: the bill tracks your fills, smoothed over the months left.

Deciding whether to join: a quick framework

The framework leaves out one personal factor that matters: whether a predictable bill from your plan is easier for your household to manage than an unpredictable charge at the pharmacy. Neither option changes the total.

Signing up, and what happens to a fill on the day you ask

You opt in through your drug plan, not through Medicare or the pharmacy. The CMS final part one guidance sets processing deadlines that matter if a costly prescription is waiting.

• During the year: the plan must process your request within 24 hours and tell you whether it is approved and when you start. Fills after that point are billed through the program.
• An urgent fill you already paid for: if you paid the pharmacy for a prescription because you believed waiting the 24 hours could seriously jeopardize your life, health, or ability to regain maximum function, you can ask for a retroactive start. The request must come within 72 hours of when that claim was processed. Once your election takes effect, the plan must refund what you paid for that fill, and for covered fills between it and your start date, within 45 calendar days, and move those costs into the monthly billing.
• Incomplete requests: if the plan needs more information, you have 21 calendar days to provide it. A denial must come with the reason and a description of the grievance process.

The first scenario above assumed you joined before the year started. In that case the first-month formula applies to your first month of coverage, with 12 months to spread over and nothing yet paid toward the threshold, which is why the January bill in that case is capped at $200.

If you change drug plans during the year

A balance you built up under one plan stays with that plan. According to the CMS final part one guidance, if you switch plans mid-year or CMS reassigns you, the old plan keeps billing you monthly for costs you incurred while you were in its program, and the new plan does not enroll you automatically. You can opt in again with the new plan, and a new plan from a different parent organization cannot refuse you because you owe the old plan. A new plan owned by the same parent organization may refuse for that year while a past-due balance is unpaid.

In practice, a mid-year switch can mean two monthly bills for a while: one from the old plan for the old balance and one from the new plan for new fills. Keep both statements, because each plan only knows its own part of your year.

Disputing an amount

If you think the cost sharing behind a monthly bill is wrong, for example because a drug was charged at the wrong tier, CMS requires plans to handle that through their standard Part D coverage determination and appeal procedures. Complaints about how the program itself was run, such as a late notice or a mishandled payment, go through the plan's grievance process. In both cases, start with the plan's member services number and ask for the decision in writing.

Missed payments, leaving early, and the next year

The rules for what happens when a bill goes unpaid are where the program differs most from a credit product, and they come from the CMS final part one guidance.

• No fees or interest. Plans may not charge late fees, interest, or fees for a particular payment method.
• A grace period of at least two months. If you miss a due date, the plan must allow at least two months to pay the overdue amount and remain in the program.
• Removal after the grace period. If the overdue amount is still unpaid, the plan ends your participation. It keeps billing what you owe, in monthly amounts no larger than the formula allows. From then on, you pay new costs at the pharmacy.
• Reinstatement. The plan must reinstate you if you show good cause for missing the payment and pay what is overdue. It may also reinstate you at its discretion if you pay.
• Your drug coverage is safe. A plan cannot disenroll you from Part D, or refuse future Part D enrollment, because you did not pay a payment-plan bill. Unpaid premiums are a different matter and follow the plan's premium rules.
• The following year. A plan may refuse to let you opt in the next year only while you still owe it an overdue balance, and only within plans from the same parent organization. Once the balance is paid, it must let you join.

Leaving voluntarily works the same way as removal, without the overdue step: you tell the plan you want out, it sends a notice, you pay the pharmacy for new fills, and it continues to bill what you already owe in capped monthly amounts. It may offer a lump-sum payoff, but CMS says it may not require one.

Participation carries into the next year on its own. Under 42 CFR 423.137(d)(9), your plan must automatically renew your participation for later plan years, notify you of the renewal, and remind you that you can opt out at any time. If you no longer want monthly billing in the new year, use the plan's opt-out process after you receive that notice.

Checking a bill from the program

A monthly bill from a payment plan is still a bill, and it is worth checking against your own records the same way you would check a medical bill. Three numbers should line up: the new out-of-pocket costs the plan says you incurred this month, which should match your fills; the balance carried from earlier months; and the months left in the year. With those three, you can reproduce the bill with the formula above. If your number and the plan's differ by more than rounding, call the plan with the fill dates and ask for the calculation in writing.

Two details trip people up. A 90-day supply is counted in the month you fill it, not spread over three months, according to CMS. So a single 90-day fill raises that month's new costs, and the bill, more than a 30-day fill would. And the payment plan does not change how quickly you reach the $2,400 cap: CMS states that opting in does not affect how you move through the Part D benefit or what counts toward the threshold. The cap is reached on the date of the fill that crosses it, even though you will still be paying for that fill in later monthly bills.

Pharmacy and plan statements are the drug-cost equivalent of the medical paperwork covered in our guides to checking an itemized bill and reading an explanation of benefits. The habit is the same: match each charge to an event, then match the total to the rule.

A one-page worksheet for estimating your own bills

You can predict your bills before opting in with four inputs and the formula. Write them down for the year you are planning:

1. The threshold: $2,400 for 2027.
2. Already paid toward it: zero if you join before January; otherwise the year-to-date out-of-pocket total from your plan statement.
3. Months left: count the month you join as the first.
4. Expected costs by month: from your current fills, noting any 90-day supplies, which count in the month they are filled.

The first-month cap is input 1 minus input 2, divided by input 3. Your first bill is that cap or that month's costs, whichever is lower. From there, carry the unpaid balance forward, add each month's new costs, and divide by the months left. If the largest bill in your worksheet is still more than you can manage, that is useful to know before January: it is the point at which to check Extra Help eligibility and to ask your plan about lower-cost covered alternatives, a conversation that belongs with your prescriber and the plan.

How this fits with the rest of your health costs

The payment plan covers Part D drugs only. Hospital and doctor bills under Part B, and hospital stays under Part A, are billed by providers on their own schedules; our guide to observation status versus inpatient status explains one of the larger Medicare cost questions on that side. Pre-tax money from a health savings account built before Medicare can pay qualified drug costs, as our HSA and FSA guide explains, and that is true whether you pay the pharmacy or pay the plan's monthly bill. If a monthly bill is hard to meet, a hospital's financial assistance policy will not help with it, because the bill comes from your drug plan; the plan's grace period and the Extra Help program are the relevant levers. The prescription cost topic page lists every drug-cost guide on this site.

Short answers on monthly drug billing

Documents behind this guide

• Social Security Act section 1860D-2(b)(2)(E), the statutory monthly cap formula, as set out and applied in CMS, Medicare Prescription Payment Plan: Final Part One Guidance (February 29, 2024), sections 30 (calculations), 40 (no fees or interest), 60.2.3 ($600 likely-to-benefit threshold), and 80 (voluntary and involuntary termination, two-month grace period, reinstatement, preclusion, no Part D disenrollment). Checked October 22, 2026.
• Electronic Code of Federal Regulations, 42 CFR 423.137(d)(9): automatic renewal for later plan years, renewal notice, and reminder of the right to opt out. Checked October 22, 2026.
• CMS, Medicare Prescription Payment Plan program page, listing the final guidance documents, which the page says apply to the first program year, 2025, and the CY2026 rule; no 2027 guidance update was listed when checked. Checked October 22, 2026.
• CMS, Fact sheet on the final part one guidance: $0 at the pharmacy, no reduction in total costs, Extra Help more advantageous for eligible enrollees. Checked October 22, 2026.
• CMS, CY2027 Rate Announcement (April 6, 2026), 2027 out-of-pocket threshold of $2,400. Checked October 22, 2026.
• Medicare.gov, Medicare Prescription Payment Plan: no cost to participate, bill from the plan instead of paying the pharmacy, does not lower drug costs. Checked October 22, 2026.

Before deciding whether spreading costs helps, estimate the total itself: our walk-through of what counts toward the 2027 Part D cap shows how quickly a steady prescription bill reaches $2,400.