Peptide reconstitution: how the math actually works

Why peptides come as powder

Almost every peptide ships as a freeze-dried (lyophilized) cake or powder inside a small glass vial. The reason is stability: peptides hydrolyze and degrade in solution but can sit dry for months or years. To use one, you add a sterile diluent — most commonly bacteriostatic water for injection — and turn the powder back into a solution.

This is the same procedure used in every hospital pharmacy when reconstituting a lyophilized drug. The difference is that the pharmacist has a label that says "add 4 mL of saline; final concentration 5 mg/mL." Your vial typically doesn't. The diluent volume is up to you, and that volume determines every dose you'll draw from it.

The only formula you need

There is exactly one piece of math behind every peptide dose:

Concentration = Vial mass ÷ Diluent volume

That's it. If you have a 5 mg vial and you add 2 mL of bacteriostatic water, the concentration is 5 ÷ 2 = 2.5 mg/mL. If you instead add 1 mL of water, the concentration doubles to 5 mg/mL.

From the concentration, every dose you draw is:

Volume to draw = Dose ÷ Concentration

If you want to draw 250 mcg from a 2.5 mg/mL solution, that's 0.25 ÷ 2.5 = 0.1 mL. On a 1 mL insulin syringe (a U-100), 0.1 mL corresponds to the 10-unit mark.

Everything below is just unit conversion and practical advice. The two formulas above are the whole game.

Choosing your diluent volume

Most peptides do not specify a required reconstitution volume — you choose, and you choose for ergonomic reasons. The two trade-offs:

More water (lower concentration) means each dose is a larger volume on the syringe, which is easier to measure precisely and easier to see. Good for very small doses.
Less water (higher concentration) means each dose is a tiny volume, which can be hard to draw accurately but lets you fit more doses in a smaller vial.

A reasonable default for most subcutaneous peptides: add enough water that a single typical dose lands somewhere between 10 and 30 units on a 1 mL insulin syringe. That range is easy to read and easy to repeat. Below 5 units, small measurement errors start to matter; above 50 units, you're wasting syringe travel.

A practical rule: for a 5 mg vial, 2 mL of water is a good default. For a 10 mg vial, 2 to 3 mL. For a 2 mg vial of something like semaglutide or tirzepatide, 1 mL is conventional.

Bacteriostatic water vs sterile water vs saline

Three diluents come up:

Bacteriostatic water for injection (BAC water). Sterile water containing 0.9% benzyl alcohol as a preservative. The benzyl alcohol allows the same vial to be entered repeatedly without bacterial growth, which is what you want for a multi-dose peptide vial. This is the default for peptide reconstitution.
Sterile water for injection. No preservative. Once the seal is broken, the contents should be used immediately or discarded.
Bacteriostatic sodium chloride. Less common; appropriate when a buffered isotonic solution is preferred.

Benzyl alcohol carries a warning for neonates (the "gasping syndrome") and is best avoided in pregnancy and very small children at large total volumes. For an adult drawing a few units per day, the total benzyl alcohol exposure is trivial — orders of magnitude below the threshold of concern in the FDA labels.

A 30 mL bottle of bacteriostatic water purchased from a US compounding pharmacy is the most common form factor. It will reconstitute many vials before it expires (28 days after the first puncture per USP guidance, regardless of how much remains).

How to actually do it

Wash your hands. Wipe both vial stoppers (the peptide and the water) with a fresh alcohol swab. Let them dry.
Draw your diluent. Pull back the plunger on an insulin syringe to the volume of water you want — for a 5 mg peptide vial, this is typically 2 mL (on a 3 mL syringe with mL markings, since 1 mL insulin syringes don't go that high).
Inject the water slowly down the side of the vial. Aim the stream at the inside glass wall rather than directly onto the powder cake. This prevents foaming, which can denature the peptide.
Do not shake. Gently swirl the vial until the powder dissolves. Most peptides go into solution in under a minute. If anything remains undissolved after a few minutes of gentle swirling, the peptide may be damaged.
Label the vial. Date of reconstitution and the resulting concentration (e.g., "2.5 mg/mL"). Memory is unreliable; labels are not.
Refrigerate. Once reconstituted, most peptides are stored at 2–8 °C. Many manufacturers publish stability data of 2–4 weeks for the reconstituted form.

Reading an insulin syringe

A standard U-100 insulin syringe has markings from 1 to 100 units. One unit equals 0.01 mL. So:

10 units = 0.1 mL
25 units = 0.25 mL
50 units = 0.5 mL

The U-100 designation comes from insulin: in a U-100 insulin solution, 1 unit of insulin equals 0.01 mL. For non-insulin peptides, the syringe doesn't measure "units of peptide" — it measures hundredths of a milliliter. The peptide content depends entirely on the concentration you reconstituted to.

Worked examples

A 5 mg BPC-157 vial reconstituted with 2 mL of bacteriostatic water

Concentration: 5 ÷ 2 = 2.5 mg/mL (= 2,500 mcg/mL)
To draw 250 mcg: 250 ÷ 2,500 = 0.1 mL = 10 units
To draw 500 mcg: 500 ÷ 2,500 = 0.2 mL = 20 units
Total doses available at 250 mcg: 5,000 mcg ÷ 250 mcg = 20 doses

A 2 mg compounded semaglutide vial reconstituted with 2 mL

Concentration: 2 ÷ 2 = 1 mg/mL
To draw 0.25 mg (the starting dose): 0.25 ÷ 1 = 0.25 mL = 25 units
To draw 0.5 mg: 50 units
Total doses available at 0.25 mg: 8 doses

A 10 mg ipamorelin vial reconstituted with 2 mL

Concentration: 10 ÷ 2 = 5 mg/mL (= 5,000 mcg/mL)
To draw 200 mcg: 200 ÷ 5,000 = 0.04 mL = 4 units
(At 4 units the reading gets imprecise — consider adding more water, say 3 mL → 3.33 mg/mL → 200 mcg = 6 units.)

Common mistakes

Treating "units" as peptide content. A 10-unit draw of one solution is not the same dose as a 10-unit draw of another. Units on the syringe measure volume, not mass.
Shaking instead of swirling. Aggressive agitation produces foam, which is bubbled-up denatured protein. Some peptides are more sensitive to this than others; the safe default is always to swirl.
Storing reconstituted vials at room temperature. Half-life in solution drops dramatically. Refrigerate as soon as the powder is dissolved.
Reconstituting more than you'll use before stability runs out. If the published stability for the reconstituted form is 28 days and you only inject twice weekly, a small vial reconstituted at low concentration may expire before you finish it.
Skipping the label. Three vials in the fridge a month from now will all look identical. Date and concentration on every one.

When to use a different approach

A small fraction of peptides require something more careful than standard reconstitution. Some examples:

Cerebrolysin and a few others come pre-reconstituted in ampules; no math required, just verify the volume on the label.
NAD+ is often supplied as a solution at high concentration; reconstitution from powder requires careful pH-buffered diluents.
HCG is conventionally measured in international units rather than mass; a 10,000 IU vial reconstituted with 1 mL becomes 10,000 IU/mL.

For these, follow whatever the manufacturer documentation specifies rather than the general formulas above.

What Vial automates

When you add a peptide entry in Vial, you tell it the vial mass, the diluent volume, and the syringe you're using. It computes the concentration once and shows you the unit count for every dose you log thereafter. The math doesn't change; the difference is that you stop doing arithmetic with sleep-deprived hands at 6 a.m. before a workout.