For most vial sizes the question never comes up, because the vial is empty first. Nothing on it expires on a date, and reconstitution starts the clocks that matter. The 28-day rule only bites when you bought more than four weeks of product in one container.
When it does bite, it is expensive. A 60 mg tirzepatide vial used at 10 mg a week has 20 mg left on day 28, worth about $93 at median prices. A 30 mg semaglutide vial at the label maintenance dose has 20.4 mg left, worth about $139.1
So the answer has two halves. The rule is about contamination, not potency. And the reason you are facing it at all is usually the vial size you chose.
The arithmetic, at median prices from our September capture:1
- Tirzepatide 10 mg vial at 10 mg weekly: gone in a week, rule never applies
- Tirzepatide 30 mg vial: three weeks, rule never applies
- Tirzepatide 60 mg vial: six weeks of product, about $93 discarded at day 28
- Semaglutide 20 mg vial at 2.4 mg weekly: 8.3 weeks of product, about $71 discarded
- Retatrutide 60 mg vial at 8 mg weekly: 7.5 weeks of product, about $189 discarded
What the 28 days is actually measuring
Not the molecule. Contamination risk in a container you keep putting needles into.
The figure comes from compounding standards for multi-dose vials containing a preservative, and preservative is the operative word: bacteriostatic water contains benzyl alcohol, which holds bacterial growth down for a period, not forever. The number is a conservative default for a vial being entered repeatedly under imperfect conditions.
Two things follow from that.
It is a proxy for punctures, not a property of the drug. A vial entered twice a week is in a different state at day 28 from one entered daily, and the calendar cannot tell them apart. Our puncture guide covers what actually accumulates.
And it is not a universal number. The same molecule carries different in-use periods in different products: an approved semaglutide pen is labelled for 56 days after first use, while other products in the same class carry 28. Those numbers come from stability and preservative testing on a specific formulation, not from a general law about peptides. Our storage guide covers where each figure comes from.
What actually limits how long a vial lasts?
Three separate clocks run at once after reconstitution, and different ones bind for different people.
| Clock | What it measures | What stops it |
|---|---|---|
| Preservative | How long benzyl alcohol keeps bacterial growth down | Calendar days after mixing |
| Molecule | How much intact compound is left | Temperature and time, gradually |
| Stopper | Rubber integrity under repeated entry | Number of punctures |
Which one you meet first is decided by your vial size and your dosing frequency, not by anything on the label.
| At label maintenance doses | Weeks of product | Weekly draws | Which limit arrives first |
|---|---|---|---|
| Tirzepatide 10 mg | 1.0 | 1 | Empty first, no limit reached |
| Tirzepatide 30 mg | 3.0 | 3 | Empty first |
| Tirzepatide 60 mg | 6.0 | 6 | 28 days, with the vial two thirds used |
| Semaglutide 10 mg | 4.2 | 4 | 28 days, just |
| Semaglutide 30 mg | 12.5 | 12 | 28 days, with most of it left |
| Retatrutide 30 mg | 3.8 | 4 | Empty first |
| Retatrutide 60 mg | 7.5 | 8 | 28 days |
Calculated from median vial sizes and label maintenance doses.1
The stopper clock is the one nobody counts. Vial stoppers are typically validated to a small number of punctures, far fewer than a daily regimen puts through them, which our puncture guide covers in detail. On a weekly compound the draws are few enough that this rarely binds, and on daily compounds it binds long before the calendar does.
Does the peptide itself go bad at 28 days?
Usually not, and this is the part the rule was never about.
Degradation in solution is driven by temperature, pH and time, and it is gradual. A vial in the fridge does not become inert on day 29, and nothing about it expires the way a date on a carton does. What changes with time is how much of the original compound is still intact, and nothing visible tells you where you are on that curve.
That is why "it looks fine" is not evidence. A solution can be clear and have lost potency, and it can also be contaminated without looking different. Cloudiness tells you something has gone wrong; clarity tells you nothing.
Can I get an extra week out of my vial?
Plenty of people do, and the risk is not evenly distributed.
It depends on which of the three clocks above you have been running down fastest. Someone drawing once a week from a vial kept steadily cold has used little of the stopper and little of the preservative. Someone drawing daily, or leaving the vial on a counter between doses, has used a lot of both.
The diluent decides the starting point. Bacteriostatic water buys you the 28 days in the first place, and a vial mixed with sterile water has no such protection and a much shorter usable window.
None of that gives you a number. Anyone telling you 42 days is safe invented it, with the same confidence as the person telling you 29 is dangerous.
Is a bigger vial still worth buying?
Only if you finish it. Two pieces of standard advice collide here.
Buying larger is cheaper per milligram. A 60 mg tirzepatide vial runs about $2.20 per mg against $4.75 for a 20 mg vial, so the large vial looks like a 50% saving.1 That advice is sound and we publish it.
But a 60 mg vial at 10 mg a week is six weeks of product in a container the rule says to discard at four. Take the rule seriously and the saving reverses: you paid for 60 mg and used 40.
| Vial | Weeks of product at label dose | Left at day 28 | Value discarded |
|---|---|---|---|
| Tirzepatide 20 mg | 2.0 | none | nothing |
| Tirzepatide 30 mg | 3.0 | none | nothing |
| Tirzepatide 60 mg | 6.0 | 20 mg | about $93 |
| Semaglutide 10 mg | 4.2 | 0.4 mg | about $3 |
| Semaglutide 20 mg | 8.3 | 10.4 mg | about $71 |
| Semaglutide 30 mg | 12.5 | 20.4 mg | about $139 |
| Retatrutide 60 mg | 7.5 | 28 mg | about $189 |
At median prices, using label maintenance doses as the divisor.1
The decision that removes the problem is made before you buy: choose a vial size that runs out in about four weeks at your dose. At that point the rule costs you nothing, and the per-mg saving on the larger vial was never real.
Why don't sellers say how long a vial lasts?
Because nothing on a listing is set up to tell you.
A vial listing gives a size and a price. It does not tell you how many weeks that is at any dose, and it does not mention that the largest size may be the worst value once storage limits are applied. The cost-per-mg comparison that makes big vials look cheap is the only arithmetic on the page.
Our dose calculator does the other half: how long a given vial lasts at a given dose, which is the number that decides whether the 28-day question ever reaches you.
What should I do with the leftover?
There is no good answer, which is why the buying decision matters more.
You cannot give it away. Passing on a mixed vial means passing on your storage history, your puncture count and your mixing technique, which nobody can assess from the outside.
You cannot sell it, and a seller offering half-used vials is telling you something about everything else they sell.
So it is disposal, and the vial half of that is messier than the sharps half, which our disposal guide covers.
The practical version: work out how many weeks a vial will last you before you buy it, and pick the size that lands near four. That is one calculation, done once, and it removes this entire question.
Common questions
Do I have to throw away my vial after 28 days?
For most vial sizes the vial is empty before then. The rule applies to what remains in a multi-dose vial mixed with bacteriostatic water, and it is about contamination risk from repeated punctures, not the peptide going bad.
Can I use my tirzepatide vial for an extra week?
Many people do. The risk depends on how many times the vial has been entered, how consistently it has been refrigerated, and whether it was mixed with preserved water. None of those gives you a safe number of extra days, and nobody can give you one honestly.
Does the peptide lose potency at 28 days?
Not at a point. Degradation is gradual and driven by temperature and time, and a clear solution tells you nothing about how much intact compound is left.
Why do some products say 56 days?
Because in-use periods come from testing a specific formulation, not from a rule about peptides. An approved semaglutide pen is labelled for 56 days after first use while other products in the class carry 28.
Is a bigger vial still better value?
Only if you use it. A 60 mg tirzepatide vial is about half the price per mg of a 20 mg vial, and it also holds six weeks of product against a four-week storage limit. Discard the remainder and the saving disappears.
How long is a peptide good for after reconstitution?
For a vial mixed with bacteriostatic water and kept refrigerated, 28 days is the usual figure, and it describes contamination risk rather than an expiry date for the compound. With sterile water the window is much shorter. With most vial sizes you finish the vial before either matters.
Do peptides expire after mixing?
Not on a date, in the way food or a sealed medicine does. What happens is gradual: the preservative's protection runs down, the molecule degrades slowly with time and temperature, and the stopper wears with each puncture. The 28-day figure is a conservative line drawn across that, not a moment when something changes.
Which comes first, 28 days or running out?
Depends entirely on vial size. At label maintenance doses a 10 or 30 mg tirzepatide vial is empty before day 28, while a 60 mg vial still has 20 mg in it. Semaglutide vials above 10 mg all reach the date with product left.
Can I give my leftover peptide to someone else?
No. A mixed vial carries your storage history, your puncture count and your mixing, none of which anyone else can assess. The same reasoning applies to buying a part-used vial from anyone.
What if I mixed it with sterile water instead?
Then the 28 days does not apply, and the window is much shorter, because sterile water has no preservative to hold bacterial growth down. Our guide on the two waters covers the difference.
Does freezing a mixed vial extend it?
Freezing and thawing stresses peptides and is not a way to extend a preserved vial's in-use period. The contamination clock is not paused by cold.
How do I know if it has gone bad?
You often cannot. Cloudiness, particles or a change in colour mean stop. Looking normal is not evidence that it is fine, which is the uncomfortable part of the whole question.
Sources
- Peptide Decoding vendor price capture, 23 September 2026. peptidedecoding.com/prices. Our own data, counting one priced product at one seller as a listing: 6,495 listings across 133 sellers. Median prices for in-stock single vials with exact prices and sizes in milligrams, multipacks excluded. Weeks of product and value discarded are calculated from the median price per mg and the label maintenance dose for each compound, and describe the cost of powder rather than of a prescription.
Citing this page. Peptide Decoding. Do I Have to Throw My Vial Out at 28 Days? Vial economics from the 23 September 2026 capture. https://peptidedecoding.com/guides/vial-28-days