How much water should I add to reconstitute a peptide?

There is no single correct amount. The water sets the concentration: less water makes a stronger mix, more water makes a weaker one that reads as more units on the syringe. A common approach is to choose a water volume that makes your usual dose land on an easy-to-read number of units. If your dose comes to less than one unit, add more water.

Can I shake the vial to dissolve the powder faster?

The usual advice is not to shake. Let the water run down the side of the vial and swirl gently, then let it sit. Peptides are delicate and vigorous shaking is commonly avoided.

What if my dose is less than one unit on the syringe?

A syringe cannot accurately draw less than one unit. Add more water so the same dose reads as a larger, readable number of units, then recalculate. Vialog will not give you a number a syringe cannot draw.

How to reconstitute a peptide, step by step

Q: Why bacteriostatic water and not tap or sterile water?

Bacteriostatic water contains a small amount of benzyl alcohol that limits bacterial growth, so a multi-dose vial can be used over several days. Sterile water has no preservative. This is general information; follow the guidance of a qualified healthcare professional.

Reconstitution is the step where a vial of dry, freeze-dried (lyophilised) peptide powder becomes a liquid you can measure and inject. You do it by adding bacteriostatic water to the vial. That part is easy. The part that trips people up is what comes after: turning milligrams in a vial into units on a syringe — and there are three conversions in between, each one a place to be wrong by a factor of ten.

This guide walks all three, slowly. It is general information about the arithmetic and the mechanics, not a dosing recommendation — your dose comes from a qualified healthcare professional, not from a web page.

What you'll have in front of you

A vial of powder, labelled with a number of milligrams (for example, 5 mg or 10 mg).
A vial of bacteriostatic water — the preservative in it is what lets a vial be used over several days.
An insulin syringe graduated in units. Most are U-100, where 1 ml is 100 units.
Alcohol swabs for the stoppers.

Step 1 — Know your three numbers

Before you touch anything, write down three things:

What's in the vial — the mg of powder printed on the label.
The water you'll add — how many ml of bacteriostatic water you plan to put in.
Your dose — usually written in micrograms (mcg) or milligrams (mg).

These three decide everything else. Notice that the amount of water is your choice: it sets how concentrated the solution is, and therefore how many units your dose comes to. We'll come back to that.

Step 2 — Add the water

Swab both stoppers. Draw your chosen volume of bacteriostatic water into the syringe, then let it run slowly down the inside wall of the vial onto the powder — not straight down onto it. Swirl the vial gently to help it dissolve. Don't shake it. Peptides are delicate; the usual advice is a gentle swirl and a little patience while it goes clear.

Step 3 — Work out the concentration

Concentration is simply how much peptide is in each millilitre of the finished solution:

concentration (mg/ml) = mg in the vial ÷ ml of water added

Add 2 ml of water to a 5 mg vial and you have 2.5 mg/ml. Add 1 ml instead and the same powder is twice as strong: 5 mg/ml. Same peptide, different number of units per dose — which is why the water volume matters as much as it does.

Step 4 — Work out the volume of one dose

Now turn your dose into a volume. Make sure the dose and the concentration are in the same unit first — if your dose is in micrograms, divide by 1000 to get milligrams (250 mcg = 0.25 mg). Then:

volume (ml) = your dose (mg) ÷ concentration (mg/ml)

A 0.25 mg dose at 2.5 mg/ml is 0.1 ml.

Step 5 — Convert the volume to units

This is the conversion nobody labels, and the one that causes the most trouble. A syringe isn't marked in millilitres you can eyeball — it's marked in units. On a U-100 syringe:

units = volume (ml) × 100

So 0.1 ml is 10 units. That's the mark you draw to.

Worked all the way through: a 5 mg vial, 2 ml of water, a 250 mcg dose → 2.5 mg/ml → 0.1 ml → draw to 10 units on a 1 ml U-100 syringe.

Step 6 — If the answer is under one unit, add more water

Sometimes the maths gives you a number a syringe physically can't draw — say 0.8 of a single unit. There's no honest way to draw eight-tenths of one unit, and rounding it is exactly how a dose ends up wrong. The fix isn't a smaller number; it's more water. Diluting the same dose into a larger volume moves it up to a readable number of units. This is the one rule most calculators skip — and the reason Vialog refuses to print a number your syringe can't draw.

Storing what's left

Once reconstituted, most people keep the vial in the fridge and use it within the shelf life of the bacteriostatic water — often quoted as around 28 days, but follow the specific guidance you were given. Keep it away from light and heat, and don't freeze a reconstituted solution.

This article is general educational information about arithmetic and mechanics. It is not medical advice and not a dosing recommendation. Vialog is a calculator and a log, not a medical device, and does not tell you what to take. Always consult a qualified healthcare professional.

Let the maths happen in front of you.

Type in your vial, your water and your dose. Vialog does all three conversions, shows the working, and won't hand you a number you can't draw.

See Vialog — coming soon

Frequently asked

How much water should I add?

There's no single right amount — the water sets the concentration. Less water = stronger mix and fewer units per dose; more water = weaker mix and more units. A common approach is to pick a water volume that lands your usual dose on an easy, readable number of units. If a dose comes to less than one unit, add more water.

Why bacteriostatic water specifically?

It contains a small amount of benzyl alcohol that limits bacterial growth, so a multi-dose vial can be used over several days. Sterile water has no preservative. Follow the guidance you were given on which to use.

Can I shake the vial to speed it up?

The usual advice is no — swirl gently and wait. Vigorous shaking is commonly avoided with peptides.

What if my dose is less than one unit?

Add more water so the same dose reads as a larger, drawable number of units, then recalculate. See the worked example for exactly how that plays out.

Keep reading: Milligrams, micrograms and units — the conversion that trips everyone.