How to reconstitute a peptide

The arithmetic, in three steps

Reconstitution math is two divisions and one multiplication. Work through it once by hand and you will never be confused by a vial label again.

1. Find the concentration

A vial holds a fixed amount of peptide, usually stated in milligrams. Adding bacteriostatic water does not add or remove any peptide — it only spreads the same amount through a volume. Concentration is the total amount divided by the water you added.

5 mg = 5000 mcg ÷ 2 mL = 2500 mcg per mL

2. Find the volume for your dose

Divide the dose you're working with by that concentration.

250 mcg ÷ 2500 mcg/mL = 0.1 mL

3. Convert to syringe units

Standard insulin-style syringes are marked in units, where 1 mL equals 100 units on any of them (more on this below). Multiply the volume by 100.

0.1 mL × 100 = 10 units

You can do this arithmetic by hand every time, or use Peptra's free reconstitution calculator, which runs the same three steps on the numbers you enter, in your browser, and tells you if a draw is too small to measure accurately or too large for the syringe you picked.

U-100 vs. U-50: they read the same

This is the single most common point of confusion in reconstitution, and it goes both ways — people assume a U-50 syringe reads "half" of a U-100 for the same draw, or that switching syringes changes the dose. Neither is true.

	U-100	U-50	U-30
Capacity	1 mL	0.5 mL	0.3 mL
Marks on the barrel	100	50	30
Units per mL	100	100	100

Every column in that last row is the same number. A U-50 barrel is simply shorter and holds less liquid — it is not scaled differently. A draw of 0.1 mL is 10 units whether you draw it on a U-100 or a U-50 syringe. What changes between them is only how much volume the barrel can physically hold, which matters for whether your total draw fits, not for what number it reads.

The under-two-units problem

On a standard barrel, the physical space between unit marks is small — often under a millimeter. When a dose works out to one or two units, a slight misread of the plunger is a large percentage error on a very small amount. There is no way to read that precisely by eye.

The fix is dilution, not a different syringe: reconstitute with more bacteriostatic water. The total peptide in the vial does not change — it just spreads across a larger volume, so the same dose draws to a larger, more legible number of units.

Vial	Water	Concentration	250 mcg draws to
5 mg	1 mL	5000 mcg/mL	5 units
5 mg	2 mL	2500 mcg/mL	10 units
5 mg	3 mL	1667 mcg/mL	15 units

All three rows deliver exactly the same amount of peptide for that dose. The third row is simply easier to draw accurately, because 15 marks give more room for precision than 5 do.

Technique

A few habits reduce contamination risk and protein damage regardless of what you're reconstituting:

• Wipe both rubber stoppers — the water vial and the peptide vial — with an alcohol swab before inserting a needle.
• Inject the water down the inside wall of the vial, not directly onto the powder. A direct stream can shear and denature the protein.
• Swirl gently; don't shake. Vigorous shaking can denature a peptide through mechanical stress and introduces foam that makes the fill level hard to read.
• Let it fully dissolve before drawing a dose. Cloudiness, visible particles, or a haze that doesn't clear are reasons to not use that vial.
• Draw air into the vial before you draw liquid out — pushing an equal volume of air in first equalizes pressure and makes an accurate draw easier.

Storage and how long a reconstituted vial is good for

This is the question with the least satisfying honest answer, and most pages give you a confident number anyway. Here is what is actually known versus assumed.

What's solid: FDA-approved products come with manufacturer stability testing behind a specific number. Ozempic's prescribing information, for example, allows a pen to be used for a defined window after first use, stored refrigerated or at controlled room temperature, discarded at the end of that window regardless of how much remains. That number exists because the manufacturer ran real stability studies to support it.

What's not solid: a peptide you reconstitute yourself, or that a compounding pharmacy prepared, was not necessarily tested the same way. USP's sterile-compounding standard (Chapter <797>) sets short default windows for compounded sterile preparations specifically because, absent product-specific stability data, the safe assumption is a short one — refrigerated defaults are commonly measured in days, not months, unless specific testing supports longer. That is a standard written for pharmacies with controlled sterile environments; it is, if anything, an optimistic reference point for a home setup with less environmental control.

Given that, the honest guidance is: refrigerate reconstituted material, keep it out of light, use it within the shortest window that any source attached to that specific product supports, and do not treat a generic online number as if it came from testing on your specific vial. Watch for cloudiness, discoloration, or visible particles as a reason to discard early — but their absence is not proof a solution is still good, since a solution can lose potency without changing appearance. If in doubt, the conservative choice is to reconstitute a smaller volume you'll use up sooner rather than a large one that sits for weeks.

Related reading

• Peptide reconstitution calculator — do the math above automatically.
• Injection site rotation chart — where to inject once your dose is drawn.
• What "503A compounded" and "research-only" actually mean.
• All guides.