A 20 mg vial of tirzepatide lasts 1.3 to 8 weeks, and a 20 mg vial of retatrutide 1.7 to 10 weeks. A 3 mg vial of semaglutide lasts 1.25 to 12 weeks. It all depends on the weekly dose. Before you order, it pays to calculate the supply for the whole period, not for a single week. Below you will find dose schedules from drug labels and clinical trials, converted into weeks, number of vials and cost per week.
Where does the weekly dose for the calculation come from?
We take the weekly dose from two sources: the labels of approved drugs and published trials. Semaglutide has the schedule from the Wegovy label, and tirzepatide the one from the Zepbound label. Retatrutide is not approved yet, so its doses come from phase 2 and phase 3 trials. All three molecules are given once a week, which makes the arithmetic much simpler.
These schedules serve here only as a reference point for calculating supply. The peptides in our shop are material for laboratory research and come with no dosing recommendations. The difference between a drug and research material is explained in the guide why semaglutide and tirzepatide are prescription only.
How do you calculate supply in weeks?
Divide the content of the vial in milligrams by the weekly dose in milligrams. The result tells you how many weeks one vial lasts. With 20 mg and a 5 mg dose you get four weeks. With a 2.5 mg dose the same vial lasts eight weeks.
You calculate the cost per week in a similar way: divide the price of the vial by the number of weeks. This is the best measure for comparing options, because it takes price and content into account at the same time. Converting milligrams into marks on an insulin syringe is covered in the guide on converting a peptide dose to syringe units.
How long does a 3 mg vial of semaglutide last?
A 3 mg vial of semaglutide covers exactly the first eight weeks of the Wegovy schedule. The label starts at 0.25 mg a week for four weeks. The dose then rises to 0.5 mg, 1 mg and 1.7 mg, each for another four weeks. From week 17 the maintenance dose is 2.4 mg or 1.7 mg a week.
| Weeks of the Wegovy schedule | Weekly dose | Use in this period | Weeks from one 3 mg vial |
|---|---|---|---|
| 1 to 4 | 0.25 mg | 1 mg | 12 |
| 5 to 8 | 0.5 mg | 2 mg | 6 |
| 9 to 12 | 1 mg | 4 mg | 3 |
| 13 to 16 | 1.7 mg | 6.8 mg | 1.8 |
| from 17 | 2.4 mg | 2.4 mg per week | 1.25 |
The first 16 weeks of the schedule use 13.8 mg in total. You therefore need five vials, or 15 mg, with 1.2 mg to spare. At 90 EUR for semaglutide 3 mg from Bio-Peptide this comes to 450 EUR for four months. At the maintenance stage of 2.4 mg a week one vial lasts a week and two days.
How long does a 20 mg vial of tirzepatide last, and a 40 mg pen?
A 20 mg vial of tirzepatide lasts eight weeks at 2.5 mg and four weeks at 5 mg. The Zepbound label starts at 2.5 mg a week for four weeks. The dose is then raised by 2.5 mg, no more often than every four weeks. The maintenance doses are 5 mg, 10 mg or 15 mg, and 15 mg is the maximum.
| Weekly dose | 20 mg vial: weeks | 20 mg vial: cost per week | 40 mg Quick Pen: weeks | 40 mg Quick Pen: cost per week |
|---|---|---|---|---|
| 2.5 mg | 8 | 25 EUR | 16 | 21.88 EUR |
| 5 mg | 4 | 50 EUR | 8 | 43.75 EUR |
| 7.5 mg | 2.7 | 75 EUR | 5.3 | 65.63 EUR |
| 10 mg | 2 | 100 EUR | 4 | 87.50 EUR |
| 15 mg | 1.3 | 150 EUR | 2.7 | 131.25 EUR |
The table uses the prices of tirzepatide 20 mg from Bio-Peptide at 200 EUR and tirzepatide 40 mg Quick Pen from Nouveaux at 350 EUR. A milligram in the pen costs 8.75 EUR, and in the vial 10 EUR. At every dose the pen is therefore 12.5 percent cheaper per week.
The label schedule taken up to 10 mg uses 100 mg over the first 16 weeks. That is exactly two 40 mg pens and one 20 mg vial, 900 EUR in total. Five 20 mg vials would cost 1000 EUR, and three pens 1050 EUR with 20 mg left over.
How long do 20 mg and 40 mg of retatrutide last?
A 20 mg vial of retatrutide lasts five weeks at 4 mg and less than two weeks at 12 mg. The phase 2 trial published in the New England Journal of Medicine tested weekly doses of 1, 4, 8 and 12 mg. The 4, 8 and 12 mg groups started at 2 mg or 4 mg. The phase 3 trial TRIUMPH-1 compared weekly doses of 4, 9 and 12 mg.
| Weekly dose | Where tested | 20 mg vial: weeks | 20 mg vial: cost per week | 40 mg Quick Pen: weeks | 40 mg Quick Pen: cost per week |
|---|---|---|---|---|---|
| 2 mg | starting dose, phase 2 | 10 | 20 EUR | 20 | 17.50 EUR |
| 4 mg | phase 2 and phase 3 | 5 | 40 EUR | 10 | 35 EUR |
| 8 mg | phase 2 | 2.5 | 80 EUR | 5 | 70 EUR |
| 9 mg | phase 3 | 2.2 | 90 EUR | 4.4 | 78.75 EUR |
| 12 mg | phase 2 and phase 3 | 1.7 | 120 EUR | 3.3 | 105 EUR |
The prices in the table are for retatrutide 20 mg from Bio-Peptide at 200 EUR and retatrutide 40 mg Quick Pen from Nouveaux at 350 EUR. After 48 weeks of the phase 2 trial, body weight fell by an average of 17.1 percent at 4 mg and 24.2 percent at 12 mg. In the placebo group it fell by 2.1 percent. The phase 3 results are discussed in the guide on retatrutide in phase 3 trials.
Vial or pen: which works out better for a longer supply?
The 40 mg pen is cheaper per milligram, and the vial gives more freedom in storage. You can keep the powder in a vial and dissolve it only when you want to start. The solution in a pen is ready at once, so its shelf life runs from the purchase. At low doses, when one pen lasts a dozen or more weeks, this difference really matters.
Both pens are listed next to the vials on the tirzepatide and retatrutide pages. Storage conditions for powder and solution are described in the guide on how to store peptides.
How many vials to order for the first four months?
The number of vials for 16 weeks depends on the molecule and on how far the schedule goes. The summary below combines the calculations from the previous sections into one shopping list.
| Molecule and 16-week schedule | Use | Cheapest set | Cost |
|---|---|---|---|
| Semaglutide following the Wegovy label, up to 1.7 mg | 13.8 mg | 5 vials of 3 mg | 450 EUR |
| Tirzepatide following the Zepbound label, up to 10 mg | 100 mg | 2 pens of 40 mg and 1 vial of 20 mg | 900 EUR |
| Tirzepatide at a steady 5 mg | 80 mg | 2 pens of 40 mg | 700 EUR |
| Retatrutide at a steady 4 mg | 64 mg | 2 pens of 40 mg | 700 EUR |
Shipping abroad costs a fixed 30 EUR with prepayment by bank transfer or 80 EUR cash on delivery. Current prices of all options are collected in the price list, and the brands and variants of each molecule on the semaglutide, tirzepatide and retatrutide pages.
What else affects how much you actually use?
Three things affect actual use: tolerance of the dose, losses when drawing up and the pace of increases. The Wegovy label allows the next step to be delayed by four weeks when a dose is poorly tolerated. The supply then lasts longer, because the lower dose lasts longer. In the phase 2 retatrutide trial a lower starting dose of 2 mg reduced gastrointestinal complaints compared with 4 mg.
With vials there is also a small loss of solution in the dead space of the syringe. That is why a set with no spare at all is worth topping up with one reserve vial. Dissolving the powder step by step is described in the guide on how to reconstitute peptides.
Frequently asked questions
How many weeks does a 20 mg vial of tirzepatide last?
Eight weeks at 2.5 mg a week, four at 5 mg, two at 10 mg and just under a week and a half at 15 mg.
How long does 3 mg of semaglutide last?
It covers exactly the first eight weeks of the Wegovy label schedule, that is 0.25 mg and 0.5 mg for four weeks each.
How many weeks does a 40 mg retatrutide pen give?
Ten weeks at 4 mg a week, five at 8 mg and just over three at 12 mg.
Is a 40 mg pen cheaper than two 20 mg vials?
Yes. The pen costs 350 EUR and two vials 400 EUR, so a milligram in the pen is 12.5 percent cheaper.
How many vials of tirzepatide are needed for four months?
The label schedule taken up to 10 mg uses 100 mg, that is two 40 mg pens and one 20 mg vial.
This is information about the products and their content, not medical advice. The choice of dose belongs to a doctor.
Sources
- FDA. Wegovy (semaglutide) injection, prescribing information, 2024. https://www.accessdata.fda.gov/drugsatfda_docs/label/2024/215256s011lbl.pdf
- FDA. Zepbound (tirzepatide) injection, prescribing information, 2024. https://www.accessdata.fda.gov/drugsatfda_docs/label/2024/217806s013lbl.pdf
- Jastreboff AM, Kaplan LM, Frías JP et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity - A Phase 2 Trial. New England Journal of Medicine, 2023, 389(6): 514-526. https://doi.org/10.1056/NEJMoa2301972
- ClinicalTrials.gov. A Study of Retatrutide (LY3437943) in Participants Who Have Obesity or Overweight, TRIUMPH-1, NCT05929066. https://clinicaltrials.gov/study/NCT05929066
- Eli Lilly. Lilly's triple agonist retatrutide delivered powerful weight loss in pivotal phase 3 obesity trial, 21 May 2026. https://www.prnewswire.com/news-releases/lillys-triple-agonist-retatrutide-delivered-powerful-weight-loss-in-pivotal-phase-3-obesity-trial-302778859.html