How To Reconstitute A Peptide Vial Without Guessing

Most of the harm in this space is not pharmacology. It is arithmetic.

Someone reads a study that used 250 micrograms per kilogram in a rat, converts it wrong, draws what they think is a small dose, and injects ten times what they meant to. Or they under-dose by a factor of five for eight weeks, conclude the compound does nothing, and tell forty people online that it does nothing.

So here is the whole calculation, once, in the units your syringe is actually marked in.

The three numbers

Every reconstitution is the same three numbers.

1. Vial mass — how many milligrams of lyophilised powder are in the vial. Printed on the label. Usually 5 mg or 10 mg.
2. Diluent volume — how many millilitres of bacteriostatic water you add. This is your choice, and it is the only variable you control.
3. Concentration — vial mass ÷ diluent volume. Milligrams per millilitre.

That is it. Everything else is unit conversion.

Why the syringe is the problem

A U-100 insulin syringe is not marked in millilitres. It is marked in units, where 100 units = 1 mL. So:

• 100 units = 1.0 mL
• 50 units = 0.5 mL
• 10 units = 0.1 mL

The syringe was designed for insulin, where "units" means international units of insulin activity. For peptides the marking is purely a volume scale. Ignore the word "units" and read it as hundredths of a millilitre.

The worked example

A 10 mg vial. You add 2 mL of bacteriostatic water.

Concentration = 10 mg ÷ 2 mL = 5 mg/mL = 5000 mcg/mL
Per unit      = 5000 mcg ÷ 100 units = 50 mcg per unit

So on that vial:

You want	You draw
250 mcg	5 units
500 mcg	10 units
1000 mcg (1 mg)	20 units

Change the diluent volume and every number in that table changes. Add 1 mL instead of 2 and every unit is worth 100 mcg. Add 3 mL and every unit is worth 33 mcg. The powder does not change. Your arithmetic does.

Choosing the diluent volume

There is no correct answer, only a convenient one. Aim for a target dose that lands between 10 and 25 units, because that range is easy to read on the barrel and forgiving of a small drawing error. A dose of 2 units is a dose you will get wrong.

Practical default: for a 10 mg vial where you want 500 mcg per shot, add 2 mL and draw 10 units.

Bacteriostatic water, not sterile water

Bacteriostatic water contains 0.9% benzyl alcohol. That preservative is what lets you re-enter the vial repeatedly over weeks without culturing something. Sterile water has no preservative and is single-use. Saline is neither, and will crash some peptides out of solution.

If you can only get sterile water, you are running a single-use vial, and you should be honest with yourself about that.

Handling

• Angle the needle so the water runs down the inside wall of the vial. Do not fire it directly into the powder cake.
• Swirl. Do not shake. These are fragile chains of amino acids and mechanical shear degrades them.
• Refrigerate after reconstitution. Most reconstituted peptides are treated as good for about 30 days at 2–8 °C, less if the vial has been through temperature swings.
• Protect from light. The box it came in is a fine light shield.
• Rotate injection sites. Abdomen, flank, upper outer thigh. Alcohol prep every time. Dispose of sharps in a real sharps container.

The check that catches the bad ones

Before you draw anything, do the multiplication in the other direction. Take the number of units you plan to inject, multiply by the per-unit microgram value, and ask whether that total is inside the range you read about.

If your answer is 5000 mcg and everything you have read says 250, you have made an order-of-magnitude error. That check takes four seconds and it is the single highest-yield safety habit in this entire category.

Where the dosing cards live

Every protocol on this site carries its own reconstitution block with the vial size, diluent volume and unit count already worked out. Start at the dosing cards, then read the protocol page for the compound you actually care about.