“Every 4 weeks” and “monthly” are not the same bill. Four weeks is 28 days. Thirteen of them make 364 days. Twelve months make 365.
So a plan billed every four weeks charges 13 times a year. A monthly plan charges 12. Same printed figure, one extra charge.
Where the 28 days come from
The 28-day cycle traces back to the box. The weekly GLP-1 drugs are dosed once a week and packed four doses to a box.
The tirzepatide label states it is injected once weekly.[1] Zepbound cartons hold 4 single-dose pens or 4 single-dose vials[1]. Other cartons hold 1 vial or 1 KwikPen[1].
The multi-dose vial holds 4 doses. The KwikPen holds four weekly doses[1]. Every four-dose pack is 28 days.
The semaglutide label says the same thing in its own words. Wegovy injection is given once weekly, on the same day each week[2].
Its cartons hold 4 single-dose pens or 4 single-dose syringes. The FlexTouch pen holds 4 weekly doses of 2.4 mg[2].
| Pack | Label says it holds | Days it covers |
|---|---|---|
| Zepbound single-dose pens | Carton of 4 pens, once weekly | 28 |
| Zepbound single-dose vials | Carton of 4 vials, or of 1 | 28, or 7 |
| Zepbound multi-dose vial | 4 doses per vial | 28 |
| Zepbound KwikPen | Four weekly doses | 28 |
| Wegovy single-dose pens | Carton of 4 pens, once weekly | 28 |
| Wegovy FlexTouch pen | 4 weekly doses of 2.4 mg | 28 |
| Wegovy tablets | Bottle of 30, once daily | 30 |
The one exception in these two labels is the Wegovy tablet. It is taken once daily[2]. It ships in a bottle of 30 tablets[2]. That is a 30-day unit, not a 28-day one.
A plan that ships one box per charge bills on the box’s rhythm. For a weekly injection, that rhythm is 28 days.
The arithmetic: times 13, divided by 12
A four-week price is not a monthly price. To turn it into one, multiply by 13 and divide by 12.
That is the same as adding one-twelfth. The monthly figure comes out about 8.3% higher than the printed one.
| Billed monthly | Billed every 4 weeks | |
|---|---|---|
| Charges a year | 12 | 13 |
| Paid in a year | $3,600 | $3,900 |
| Per calendar month | $300 | $325 |
The yearly gap is exactly one charge. At the example figure, that is $300 more a year.
This site uses the same conversion. It is set out on the methodology page.
Right now 28 eligible price rows, from 12 sellers, are billed every four weeks. The monthly price boards leave those rows out, so a four-week figure is never ranked as a month.
The year that has 14 charges
Thirteen is the usual count, not a fixed one. A 365-day year is 13 cycles of 28 days plus one day.
If the first charge lands on January 1, the fourteenth lands on December 31. In a leap year, a January 2 start does the same.
Over many years, the average is 365.25 days divided by 28. That is about 13.04 charges a year.
So times 13, divided by 12, slightly understates the long-run cost. The gap is small: well under one percent.
A 30-day cycle is not monthly either
Some plans bill every 30 days. That is closer to a month, but not equal to one.
A 365-day year holds 12 full 30-day cycles plus five days. Over time, that adds up to about 12.17 charges a year.
A calendar month is the only cycle that charges exactly 12 times. Everything else drifts.
What the law says about disclosure
Online plans that renew until you stop them fall under a federal statute. It is the Restore Online Shoppers’ Confidence Act.
It requires the seller to disclose “all material terms of the transaction” clearly, before taking billing information[3].
It also requires express informed consent before charging, and a simple way to stop recurring charges[3].
The statute does not list which terms are material. So the practical step is the one above: find the cadence before you pay.
Stopping a plan, and what happens to a prepaid term, is covered in how to cancel a peptide subscription.
Where this changes a comparison
A four-week price beside a monthly one looks cheaper by about 8.3%. Converted, it may not be.
Plan length moves the printed figure too. That is a separate trap, set out in why one seller quotes four different prices.
To total a month with fees included, use the all-in monthly calculator. For how units differ across peptides, see how much peptides cost.
For how far one vial stretches, read how long a tirzepatide vial lasts.