Reconstitution & concentration math
For anyone holding a powder vial who needs the arithmetic to be right. The failure mode here is a 10× dosing error.
- 1The usual fill: 2 mL into a 3 mL vial, 4 mL into a 6 mL vial, 6 mL into a nasal spray bottle, roughly two-thirds full, leaving air to swirl in.
- 2Bacteriostatic water is the default diluent: sterile water plus 0.9% benzyl alcohol, the preservative that makes repeated draws from one vial workable.
- 3Run the water down the tilted glass wall and swirl gently; never blast the powder or shake hard.
- 4Dose in units on an insulin syringe, not millilitres. On a U-100, 1 unit = 0.01 mL, and the only mistake that really hurts is a decimal slip giving 10× the dose.
The calculator
Vial strength, diluent volume, dose you want. It returns the mark to draw to. The chips under the diluent field are shortcuts for the usual fills; any volume you type works just as well. It never suggests a dose: that number is yours.
Shortcuts for the usual fills: type any volume you like.
25 mcg per unit on a U-100. Add a dose above to get the mark to draw to.
Arithmetic only, on numbers you entered: it does not check that a dose is appropriate. Read your syringe barrel to confirm whether it is U-100 or U-40.
People dose peptides in units on an insulin syringe, not millilitres. On a U-100 syringe, 100 units = 1 mL, so 1 unit = 0.01 mL. A U-40 syringe uses a different scale (40 units = 1 mL). Reading a dose off the wrong scale is a 2.5× error. Always confirm which syringe you are holding.
Most research peptides ship as a freeze-dried powder because they are unstable in water. Reconstitution is just the step of dissolving that powder in a sterile diluent so it can be drawn into a syringe, and once you know the strength of the vial and how much liquid you add, every dose after that is fixed. There is nothing to eyeball. The three things worth getting right are the diluent (which one, and why), the technique (so you do not damage the peptide going in) and the arithmetic (so a decimal-place slip does not deliver ten times what you intended).
How much water to add
There is a default answer and a real answer. The default is a container convention, a habit that has settled because it leaves enough air space in the vial to swirl the solution properly, and because the resulting concentrations land on round numbers. It is what most people reach for without thinking about it.
| Container | Usual fill | What that gives you |
|---|---|---|
| 3 mL vial (what almost everything ships in) | 2 mL | About two-thirds full, with air left to mix in. A 5 mg vial at 2 mL is 2.5 mg/mL, 25 mcg per unit on a U-100. |
| 6 mL vial | 4 mL | The same two-thirds fill, scaled up. A 10 mg vial at 4 mL is 2.5 mg/mL, so the syringe reads identically to the example above. |
| Nasal spray bottle | 6 mL | Roughly 60 actuations at the usual 0.1 mL per spray. Nasal prep normally uses bacteriostatic saline rather than plain bacteriostatic water. |
The real answer is that the vial does not fix the volume. It is a trade-off you make. Less diluent gives a concentrated solution where a dose is only a unit or two, which is easy to under- or over-shoot. More diluent spreads the same dose across more units, easier to measure accurately, but fills more of the syringe. The peptide in the vial is unchanged either way; only the arithmetic on the barrel moves.
- A dose that lands under ~2 units is hard to measure precisely. More diluent spreads it out.
- A dose that exceeds one syringe barrel means either more than one draw or less diluent.
- Pick a round concentration where your usual dose is an easy number of units to read.
- Very small fills (~0.5 mL) leave powder stranded on the vial wall unless it is mixed thoroughly.
Bacteriostatic water and the alternatives
Bacteriostatic water is the default for a specific reason: it is sterile water with 0.9% benzyl alcohol added as a preservative, and it is supplied as a multiple-dose container that is designed to be punctured repeatedly. Everything else on this list exists to cover a case bacteriostatic water does not: a single-use prep, a mucous membrane, or a peptide that will not dissolve in plain water.
| Diluent | What is in it | Where it fits | Life once opened |
|---|---|---|---|
| Bacteriostatic water | Sterile water + 0.9% benzyl alcohol | The default for injectable peptides. The preservative is what makes drawing from one vial over weeks reasonable, and benzyl alcohol is itself a mild anaesthetic, so preserved diluents tend to sting less going in. | Multiple-dose; 28 days after first puncture is the standard discard point |
| Sterile water for injection | Water, nothing added | Single-use work, and the option when benzyl alcohol has to be avoided. No preservative means nothing is holding back contamination once the stopper is pierced. It is also hypotonic. The label warns it must not be injected until made approximately isotonic by the solute. | Single-dose; use and discard |
| Bacteriostatic 0.9% sodium chloride | Saline + 0.9% benzyl alcohol | Isotonic and preserved. The usual choice for nasal sprays, and for peptides whose suppliers specify saline. Not for fluid or sodium replacement, and not for epidural or spinal use. | Multiple-dose; 28 days |
| Preservative-free 0.9% sodium chloride | Saline, nothing added | Isotonic without benzyl alcohol: the fallback where the preservative is unwanted but tonicity still matters. | Single-dose; use and discard |
| 0.6% acetic acid (acidified water) | Dilute acetic acid in water | For the handful of peptides that will not dissolve in plain water. IGF-1 LR3 is the standard example. Typically used only for the initial dissolve, with the vial then brought up to volume with bacteriostatic water. | Per supplier |
It is not a property of the water. The standard practice for any punctured multiple-dose vial is to date it on first entry and discard it 28 days later unless the manufacturer states otherwise: a preservative slows microbial growth, it does not keep a repeatedly-pierced vial sterile forever. The reconstituted peptide has its own, usually shorter, stability window on top of that.
Bacteriostatic water and bacteriostatic saline are both labelled NOT FOR USE IN NEONATES. Benzyl alcohol has been linked to fatal “gasping syndrome” in newborns, whose metabolic pathways cannot clear it. Preserved diluents are also kept out of epidural and spinal procedures. Separately, formulation research has repeatedly shown benzyl alcohol can promote partial unfolding and aggregation of some proteins, which is one reason a few products specify a preservative-free diluent.
Benzyl alcohol is harsh on nasal mucosa, and repeated exposure is associated with irritation, dryness and burning. Intranasal preparations therefore generally use isotonic bacteriostatic saline rather than plain bacteriostatic water: same preservative, but carried in 0.9% saline that matches the tissue it is sprayed onto.
Mixing it without wrecking it
How you add the diluent matters as much as which one you picked. Aim the stream at the inside glass, not down onto the powder, and mix gently. The powder is fragile and the failure here is silent: a solution that looks fine but has been sheared or denatured.
- 1Draw the diluent first: measure the volume you want into the syringe before you touch the powder vial.
- 2Run it down the wall: tilt the vial and aim the stream at the inside glass so it trickles down, not straight onto the powder, which foams and shears the peptide.
- 3Swirl, don’t shake: roll or swirl the vial gently until it clears; hard shaking can denature fragile peptides.
Go deeperThe concentration math
The arithmetic the calculator is doing. Open it only if you want to see the working.
The concentration math
The arithmetic the calculator is doing. Open it only if you want to see the working.
The peptide dissolves evenly into whatever diluent you add, so the concentration is simply the amount in the vial divided by the volume of liquid. Add more liquid and the same dose occupies more units on the syringe; add less and it occupies fewer. Nothing else changes the concentration.
- Concentration
- mg in vial ÷ mL of diluent = mg/mL
- Worked example
- 5 mg vial + 2 mL diluent = 2.5 mg/mL
- Per unit (U-100)
- 2.5 mg/mL = 25 mcg per 1 unit (0.01 mL)
Go deeperStorage and working life
How long a vial lasts before and after you mix it.
Storage and working life
How long a vial lasts before and after you mix it.
Freeze-dried vials are relatively stable and are usually kept cold and dark until use. Once reconstituted, the clock starts: the solution is refrigerated and has a limited working life.
| State | Typical storage | Rough working life |
|---|---|---|
| Lyophilized (sealed powder) | Refrigerator; freezer for long-term | Months, per supplier |
| Reconstituted in bacteriostatic water | Refrigerated, 2–8 °C | ~2–4 weeks for many peptides |
| Reconstituted in preservative-free water | Refrigerated, 2–8 °C | Hours to a couple of days, no preservative |
A reconstituted solution should stay clear. Cloudiness, particles, or discolouration mean it should be discarded, regardless of the date.
This describes how reconstitution is done, and where it goes wrong. It is not a recommendation to do it, and it does not cover where to obtain anything. It is written on the assumption that the reader has already made that decision and would otherwise proceed without accurate information. Nothing here is medical advice: the safest version of every practice above still involves a clinician and lab work.
Where this applies
Related fundamentals
How contamination causes abscesses and cellulitis, the aseptic habits that prevent it, and what an infection looks like early.
The symptoms that override "finish the cycle": the clot, liver, pancreatic, and infection red flags that mean stop and get medical help now.
Sources & references
11 links
- Bacteriostatic Water for Injection, USP, FDA label (Pfizer)
- Bacteriostatic 0.9% Sodium Chloride Injection, USP, FDA label (Pfizer)
- Sterile Water for Injection, USP, FDA label (DailyMed)
- Preventing Unsafe Injection Practices, multi-dose vial 28-day rule (CDC)
- Benzyl alcohol attenuates the pain of lidocaine injections (Williams et al., 1994, PubMed)
- Bacteriostatic preserved saline for pain-free periocular injections: a review (Eye, 2021)
- Prevention of benzyl alcohol-induced aggregation of chymotrypsinogen by PEGylation (PMC)
- Reconstitution guide, vial volumes and headspace (Peptide Affect)
- How much BAC water to add to peptides, reference charts (Heritage Labs)
- How to Reconstitute Peptides, dosing math and technique (PeptideDeck)
- Bacteriostatic Water & Peptide Reconstitution: A Lab Guide (Adapt)