Most peptides and many hormone medicines are supplied as a freeze-dried (lyophilised) powder in a sealed glass vial. Before anything can be drawn into a syringe, that powder has to be dissolved in a suitable liquid. The liquid almost always used for multi-dose vials is bacteriostatic water: sterile water for injection containing 0.9% benzyl alcohol as a preservative. This guide covers the whole process, from choosing a diluent to calculating the concentration, drawing a dose on a U-100 insulin syringe, and storing the vial for the 28 days it can be used.
It is written for people who need the numbers to be right: patients reconstituting a prescribed medicine such as hCG or somatropin, and laboratory users preparing research peptides. The maths is simple once the units are clear, so we show the formula, worked examples and a lookup table rather than a rule of thumb. Baclabs sells bacteriostatic water and reconstitution consumables only; the peptide articles elsewhere on this site are reference content that links back here for the practical side.
What bacteriostatic water is and why it is the default diluent
Bacteriostatic Water for Injection, USP, is water prepared to the pharmacopoeial standard for injectables, with benzyl alcohol added at 0.9% (9 mg/mL) as a preservative. The United States Pharmacopeia (USP) product is labelled “for diluting or dissolving drugs for intravenous, intramuscular or subcutaneous injection” and is supplied in multiple-dose vials from which repeated withdrawals may be made [1].
“Bacteriostatic” means the preservative inhibits the growth of bacteria introduced when a needle passes through the rubber stopper. It does not sterilise a contaminated solution, but it keeps a properly handled multi-dose vial usable for up to 28 days after first puncture. That figure comes from USP General Chapter <797> and from the US Centers for Disease Control and Prevention (CDC) injection-safety guidance, both of which state that an opened multiple-dose container should be dated and discarded within 28 days unless the manufacturer specifies otherwise [2][3].
Plain Sterile Water for Injection contains no preservative. Its label describes it as “sterile, hypotonic, nonpyrogenic, and contains no bacteriostatic or antimicrobial agents”, and it is intended for single use [4]. That makes it inappropriate for a vial that will be punctured ten or twenty times over a month. The full comparison is in bacteriostatic water vs sterile water.
The ingredient is covered in why reconstitution diluents use 0.9% benzyl alcohol. One hard contraindication belongs here: the US label states “NOT FOR USE IN NEONATES”, and the European Medicines Agency (EMA) links intravenous benzyl alcohol exposure of roughly 99 to 234 mg/kg/day in premature infants to the fatal “gasping syndrome” described by Gershanik and colleagues in 1982 [1][5][6].
Reconstitution maths: the formula
Everything follows from one relationship:
Concentration (mg/mL) = mass of powder in the vial (mg) ÷ volume of diluent added (mL)
Multiply by 1,000 to convert to micrograms per millilitre (mcg/mL). A U-100 insulin syringe is graduated in “units” where 100 units = 1 mL, so each unit is 0.01 mL and:
mcg per unit = concentration (mcg/mL) ÷ 100
and to draw a given dose:
units to draw = desired dose (mcg) ÷ mcg per unit
Two points trip people up. The diluent volume is the number you control; the powder mass is fixed by the vial, so more water gives a weaker solution that is easier to measure. And syringe “units” are a volume measure, not a measure of drug; they mean nothing until you know the concentration.
Worked example 1: 5 mg vial, 2 mL bacteriostatic water
- Concentration = 5 mg ÷ 2 mL = 2.5 mg/mL = 2,500 mcg/mL
- Per unit on a U-100 syringe = 2,500 ÷ 100 = 25 mcg
- A 250 mcg dose = 250 ÷ 25 = 10 units (0.1 mL)
- A 500 mcg dose = 20 units (0.2 mL)
Worked example 2: 10 mg vial, 3 mL bacteriostatic water
- Concentration = 10 ÷ 3 = 3.33 mg/mL = 3,333 mcg/mL
- Per unit = 33.3 mcg
- A 1 mg (1,000 mcg) dose = 1,000 ÷ 33.3 = 30 units (0.3 mL)
If a dose works out below about 5 units, add more diluent so it occupies a more readable volume.
Concentration table for common vial sizes
The table covers 2 mg, 5 mg and 10 mg vials with 1, 2 or 3 mL of diluent. For medicines dosed in International Units (IU), such as hCG, replace “mg” with “IU”; the arithmetic is identical.
| Vial | Diluent added | Concentration | mcg per U-100 unit | Units for 250 mcg |
|---|---|---|---|---|
| 2 mg | 1 mL | 2 mg/mL (2,000 mcg/mL) | 20 mcg | 12.5 |
| 2 mg | 2 mL | 1 mg/mL (1,000 mcg/mL) | 10 mcg | 25 |
| 2 mg | 3 mL | 0.67 mg/mL (667 mcg/mL) | 6.7 mcg | 37.5 |
| 5 mg | 1 mL | 5 mg/mL (5,000 mcg/mL) | 50 mcg | 5 |
| 5 mg | 2 mL | 2.5 mg/mL (2,500 mcg/mL) | 25 mcg | 10 |
| 5 mg | 3 mL | 1.67 mg/mL (1,667 mcg/mL) | 16.7 mcg | 15 |
| 10 mg | 1 mL | 10 mg/mL (10,000 mcg/mL) | 100 mcg | 2.5 |
| 10 mg | 2 mL | 5 mg/mL (5,000 mcg/mL) | 50 mcg | 5 |
| 10 mg | 3 mL | 3.33 mg/mL (3,333 mcg/mL) | 33.3 mcg | 7.5 |
The most common error is confusing 0.1 mL with 1 mL, a ten-fold dosing error in either direction.
U-100 syringe conversions at a glance
- 100 units = 1.0 mL
- 50 units = 0.5 mL
- 10 units = 0.1 mL
- 5 units = 0.05 mL
- 1 unit = 0.01 mL
Insulin syringes come in 0.3 mL, 0.5 mL and 1 mL sizes; a 0.3 mL syringe with half-unit graduations gives the finest resolution. Do not use a U-40 syringe with these conversions.
Choosing a diluent: bac water, sterile water or saline
The choice depends on how the vial will be used and what the product label says.
- Bacteriostatic water is the standard for any vial that will be entered more than once. The preserved diluent is what makes 28-day multi-dose use defensible [2][3].
- Sterile water for injection is appropriate when the whole vial is used immediately, or where a manufacturer supplies it as the solvent for a single-dose product. It is hypotonic and must not be injected on its own [4].
- Sodium chloride 0.9% (normal saline) is the solvent supplied with some licensed products; Pregnyl (hCG) in the UK, for example, is supplied with a 1 mL ampoule of sodium chloride 9 mg/mL and the summary of product characteristics (SmPC) says the solution should be used immediately [7].
- Manufacturer-supplied solvents sometimes contain their own preservative. Genotropin (somatropin) comes with a metacresol-preserved solvent and is stable for 28 days at 2–8°C after reconstitution [8]; Menopur 1200 IU likewise allows 28 days at up to 25°C [9]. Use the solvent the manufacturer supplies for a licensed medicine.
Some peptides are poorly soluble in water at neutral pH and suppliers may recommend a small volume of dilute acetic acid or a buffer. That is specific to the peptide, not a reason to avoid bacteriostatic water in the general case.
Multi-dose vials need a preserved diluent. Baclabs supplies UK-stocked bacteriostatic water in 30 mL vials, which is enough for ten or more reconstitutions at 2–3 mL each within the 28-day window.
Step-by-step aseptic reconstitution
The steps below follow the aseptic principles in USP <797> and CDC guidance, scaled down for a single vial on a clean surface [2][3]. The goal is to keep every surface that touches the solution sterile.
- Prepare. Wash hands, wipe down a flat surface, and bring both vials to room temperature. Confirm the mass on the powder vial and check the bacteriostatic water is in date, clear and colourless.
- Disinfect both stoppers. Remove the flip-off caps, swab each stopper with a fresh 70% isopropyl alcohol wipe and let it dry for at least 30 seconds. Do not touch the stopper afterwards.
- Draw the diluent. With a sterile syringe and needle, inject air equal to the intended volume into the bacteriostatic water vial to keep pressure neutral, then invert and draw the measured volume.
- Add the diluent slowly. Insert the needle through the powder vial’s stopper and direct the stream against the glass wall so it runs down the side rather than jetting onto the powder. Peptides and proteins can be damaged by mechanical stress; the Genotropin SmPC states that the product should be dissolved by gently tipping the vial and warns that vigorous shaking “might cause denaturation of the active substance” [8].
- Dissolve without shaking. Let the vial stand, then swirl gently. The solution should be clear with no particles; if it stays cloudy, do not use it.
- Label and refrigerate. Write the reconstitution date, the concentration (for example “5 mg / 2 mL = 25 mcg per unit”) and the discard date 28 days later. Date the bacteriostatic water vial at first puncture. Store at 2–8°C, protected from light, and use a fresh sterile needle and syringe for every withdrawal.
Storage, shelf life and the 28-day rule
Unopened bacteriostatic water is stored at 20–25°C and carries a manufacturer expiry date [1]. Once the stopper has been punctured, the 28-day clock starts regardless of how much remains [2][3]. The same limit is applied by convention to a peptide vial reconstituted with bacteriostatic water, because the preservative is what supports repeated entry; the chemical stability of the peptide may be shorter, and a manufacturer’s stated figure takes precedence.
Refrigeration slows chemical degradation of the dissolved peptide. Do not freeze reconstituted vials; freeze-thaw cycles can cause aggregation. The full guidance is in how to store bacteriostatic water.
Sterile vials and other consumables
Sterile empty vials for injection are useful for splitting a batch or holding a diluted working solution. Insulin syringes (29–31 gauge) suit subcutaneous use; an 18–21 gauge drawing needle makes withdrawing diluent quicker. Alcohol wipes, a sharps container and a label pen complete the kit. Everything that touches the solution should be sterile and single-use.
Reconstituting licensed medicines: hCG, somatropin and fertility drugs
Licensed products come with a solvent and an SmPC or leaflet, and those instructions override anything generic. The UK Pregnyl leaflet says to dissolve the hCG in the supplied sodium chloride solvent and use it immediately [7], whereas the US label supplies a benzyl alcohol-preserved diluent and allows 60 days refrigerated [10]; the US Gonal-f multi-dose vial is reconstituted with bacteriostatic water and discarded after 28 days [11]. The difference is the preservative in the solvent, not the hormone. See reconstituting fertility and hormone medications.
The UK legal position
Bacteriostatic water is a diluent, not a medicine with a therapeutic claim. What it reconstitutes falls into three groups under the Human Medicines Regulations 2012: licensed prescription-only medicines (somatropin, hCG, follitropin, semaglutide, tirzepatide), obtained through a prescriber; unlicensed medicines, which the Medicines and Healthcare products Regulatory Agency (MHRA) permits only for an individual patient’s special clinical need under Guidance Note 14 [12]; and research chemicals sold “not for human use”. The evidence behind each compound varies enormously.
Why this site has peptide guides
People searching for bacteriostatic water are usually about to reconstitute a specific compound, and their next questions are about that compound: what the trials showed, what the licensed dose is, and what is not known. We have written evidence-led reference pages for the peptide families most often reconstituted with bac water. Each distinguishes licensed medicines with human trial data from investigational compounds and from those with animal-only or no controlled human data, and each links back here.
- GLP-1 and multi-receptor agonists: semaglutide, tirzepatide, retatrutide, cagrilintide and CagriSema.
- Regenerative peptides: BPC-157, TB-500, LL-37 and KPV.
- Growth hormone secretagogues: CJC-1295, ipamorelin, tesamorelin, sermorelin and the GHRPs.
- Muscle growth peptides: IGF-1 variants, MGF, follistatin and somatropin.
- Metabolic peptides: HGH fragment 176-191, AOD-9604, MOTS-c, SS-31 and lipotropic injections.
- Cognitive and neuro peptides: Semax, Selank, Dihexa, Epithalon and DSIP.
- Specialised peptides: GHK-Cu, melanotan and PT-141, kisspeptin, thymosin alpha-1 and others.
This cluster’s own articles: bacteriostatic vs sterile water, storage and the 28-day rule, benzyl alcohol and hCG, HGH and fertility medications.
Frequently asked questions
How much bacteriostatic water do I add to a 5 mg vial?
It depends on the dose you need to measure. Adding 2 mL gives 2.5 mg/mL, or 25 mcg per unit on a U-100 insulin syringe, so a 250 mcg dose is 10 units. Adding 1 mL doubles that to 50 mcg per unit. Choose a volume that puts your dose between roughly 5 and 50 units.
Is bacteriostatic water the same as sterile water?
No. Both are sterile water for injection, but bacteriostatic water contains 0.9% benzyl alcohol as a preservative and is packaged for multiple withdrawals over 28 days. Sterile water for injection has no preservative, is labelled for single use, and is hypotonic, so it must never be injected without a solute dissolved in it.
How long does reconstituted peptide last in the fridge?
The multi-dose convention is 28 days from reconstitution with a preserved diluent, kept at 2–8°C, mirroring the USP <797> and CDC limits for opened multi-dose containers. Chemical stability varies by peptide, so a manufacturer’s stated figure, where one exists, takes priority.
Can I use bacteriostatic water for injection on its own?
No. It is labelled “use only as a diluent or solvent”. Injected alone it is hypotonic and can cause haemolysis; the label says not to use it intravenously unless the additives make the mixture approximately isotonic. It is also contraindicated in neonates because of benzyl alcohol toxicity.
Why does the vial have to be swirled rather than shaken?
Peptides and proteins can be denatured or aggregated by the shear and foaming that shaking produces; the somatropin SmPC states that vigorous shaking may denature the active substance. Run the diluent down the glass wall, let the vial stand, and swirl gently until clear.
References
- Bacteriostatic Water for Injection, USP, prescribing information (Hospira). DailyMed, US National Library of Medicine, current label
- Crosswalk of Guidances and Standards for Managing Single- and Multi-Dose Vials (USP <797>, CDC, TJC). American Society of Health-System Pharmacists
- Preventing Unsafe Injection Practices. US Centers for Disease Control and Prevention
- Sterile Water for Injection, USP, prescribing information (B. Braun). DailyMed, US National Library of Medicine
- Questions and answers on benzyl alcohol used as an excipient in medicinal products for human use (EMA/CHMP/508188/2013). European Medicines Agency, 2017
- Gershanik J, Boecler B, Ensley H, McCloskey S, George W. The gasping syndrome and benzyl alcohol poisoning. N Engl J Med 1982;307(22):1384-8. PubMed, 1982
- Pregnyl 1500 IU and 5000 IU powder and solvent for solution for injection, UK package leaflet (Organon). UK patient information leaflet
- Genotropin 12 mg powder and solvent for solution for injection, Summary of Product Characteristics. electronic medicines compendium (emc), Pfizer
- Menopur 1200 IU powder and solvent for solution for injection, Summary of Product Characteristics. electronic medicines compendium (emc), Ferring
- Pregnyl (chorionic gonadotropin for injection, USP), US prescribing information. DailyMed, US National Library of Medicine
- Gonal-f Multi-Dose (follitropin alfa for injection), US prescribing information. EMD Serono
- The supply of unlicensed medicinal products (“specials”): MHRA Guidance Note 14. Medicines and Healthcare products Regulatory Agency
Disclaimer: This content is for educational purposes only and does not constitute medical advice. Bacteriostatic water is supplied for reconstitution of substances intended for research and for use as directed by a healthcare professional. Peptides and medicines discussed here may be unlicensed in the UK or prescription-only; consult a qualified clinician before using any medicine. Baclabs does not sell peptides.