28-Day vs. 30-Day Prescription Cost Schedules

“Per month” is a display phrase, not a calculation. For a prescription cash-cost ledger, the underlying package and supply days determine when charges occur.

This distinction is especially visible in published GLP-1 terms. Manufacturer materials can define one month of an injectable product as four single-dose pens or vials covering 28 days, while a tablet package may be described as 30 tablets for 30 days. Always record the source's exact definition for the product and form on the existing prescription.

Why twelve times a monthly price can be wrong

A 28-day fill repeats every four weeks:

13 × 28 days = 364 days

A 30-day fill repeats on a different cycle:

12 × 30 days = 360 days

Neither cycle maps perfectly onto calendar months. If a scenario begins on January 1 and runs through December 31, the number of checkouts depends on whether a fill occurs on day one, how the horizon is defined, and the exact supply cadence. The correct approach is to generate dated charges until the next fill would fall outside the horizon.

Build the schedule, then sum it

For each record, store:

• start or next-purchase date;
• supply days;
• price for each qualifying fill;
• number of introductory fills;
• introductory expiry date;
• ongoing amount;
• recurring fees with their own cadence;
• one-time fees;
• required unknown fees.

Then create the schedule:

1. Add the first checkout on the selected start date.
2. Advance by the recorded supply days.
3. At each date, test whether an introductory term still applies.
4. Add recurring fees on their own dates rather than blending them into the drug amount.
5. Stop when the next charge is outside the chosen horizon.
6. Sum integer cents, never floating-point currency.

A fictional example

Assume a published record—not a real offer—has these user-entered terms:

• first two 28-day fills: $200.00 each;
• later fills: $350.00 each;
• one-time required fee: $15.00;
• start date: January 1;
• horizon: 84 days, including the start date.

The dated schedule is:

Day	Charge	Reason
0	$215.00	first introductory fill plus one-time fee
28	$200.00	second introductory fill
56	$350.00	ongoing fill

Known total: $765.00.

If a required shipping charge is mentioned but not quantified, the ledger should say “known subtotal $765.00; required shipping unknown.” It should not call $765 all-in.

Administrative windows are not supply cadence

A purchase offer may require a refill purchase within a longer window than the package's supply days. Current Zepbound manufacturer terms, for example, can define a one-month supply as 28 days while separately requiring the next qualifying purchase within a stated number of days from delivery or receipt.

Store both values:

• Supply days drive the modeled medication checkout schedule.
• Administrative window is a condition the user may want to review before its deadline.

Do not reinterpret an administrative window as dosing advice or as permission to change when medicine is used. Treatment and refill questions belong with the prescriber and pharmacist.

Introductory terms need three checks

An introductory amount often depends on more than fill number:

1. Is this fill within the maximum introductory count?
2. Does its purchase date fall before the offer expiry?
3. Does the user-entered record say the relevant eligibility and channel conditions still apply?

If any required fact is unknown, mark the projected charge uncertain. A ledger preserves published terms; it cannot promise that a future transaction will qualify.

Compare timelines only when identity matches

Two totals are not meaningfully comparable merely because both pages contain “GLP-1,” “semaglutide,” or “tirzepatide.” Require matching product, form, dose or strength, package, and supply definition before placing schedules side by side. Even then, the ledger should show recorded cash flows rather than recommend a medicine or pharmacy.

Use the GLP-1 cash-price ledger checklist before calculating. For eligibility, fees, and expiry clauses, see how to read published offer terms.

Keep the source date visible

Current official Wegovy and Zepbound pages show why saved terms need dates: amounts can vary by form, dose, fill number, insurance status, channel, or limited period, and the manufacturers reserve the ability to change or cancel programs. A schedule generated from a saved record is a scenario based on that observation—not a guarantee of future cost.

Open RefillScope to model an exact, dated schedule from terms you have reviewed.

All dollar figures in the worked example are fictional. This article explains recordkeeping arithmetic, not medical, insurance, or financial advice.