Australian regulatory note: Many injectable peptides are not TGA-approved for human therapeutic use in Australia. This guide applies only to lawfully prescribed or pharmacy-supplied products with written instructions, not research or online peptide vials. Always follow your prescriber's specific protocol.

Some prescribed or pharmacy-supplied injectable peptides arrive as a freeze-dried (lyophilised) powder in a small glass vial. Before use, the powder may need to be dissolved in a sterile diluent specified in the written product instructions. This process is called reconstitution. Bacteriostatic water is commonly used for some multi-dose reconstitution workflows, but it is not the correct diluent for every product.

In this guide, multi-dose means repeated withdrawals for the same person under the supplied protocol. Never share vials or syringes, and use a new sterile needle and syringe for every withdrawal.

If you have a product-specific written protocol, use it as the source of truth for diluent, volume, concentration, storage, and discard date.

This guide covers what bacteriostatic water is, how reconstitution volume affects concentration, careful transfer technique, and storage checks. Always follow the written instructions supplied with your specific product.

Peptide reconstitution supplies on a kitchen bench in morning light: our bacteriostatic water vial, a 0.5 mL insulin syringe, and two unlabelled vials of lyophilised powder
Peptide reconstitution supplies on a kitchen bench in morning light: our bacteriostatic water vial, a 0.5 mL insulin syringe, and two....

1. What Is Bacteriostatic Water?

Bacteriostatic water (BAC water) is sterile water with 0.9% benzyl alcohol added as a preservative. The benzyl alcohol helps inhibit bacterial growth during multi-dose use, but it does not sterilise a contaminated vial or make repeated punctures risk-free. Do not use bacteriostatic water where benzyl alcohol is contraindicated, including for neonates or infants, or where the product instructions specify a preservative-free diluent. After first puncture, label the bacteriostatic water vial with the opening date and discard it according to the manufacturer, pharmacy, or label instructions. Aseptic technique, correct storage, and the product's discard date still matter.

Why not regular sterile water? Plain sterile water for injection contains no preservative. A preservative-free diluent vial should not be kept for repeated withdrawals once opened. If sterile water is specified for reconstitution, follow the pharmacy's beyond-use and storage instructions for the resulting vial.

Injection Supplies now stocks 10mL bacteriostatic water vials for reconstitution workflows.

2. What You Need

  • Peptide vial: Your lyophilised (freeze-dried) product. Many lyophilised products are white or off-white, but the vial should match the appearance described in the supplied product instructions.
  • Specified diluent: A sealed, in-date vial of the diluent named in your product instructions. This may be bacteriostatic water for some multi-dose products.
  • Transfer syringe: only when specified or allowed Use the sterile transfer syringe and needle specified by your pharmacy or product instructions. If those instructions specifically allow a 10ml 18G draw syringe for an appropriate larger-volume transfer, it can move fluid faster in one pass. Use it for draw/transfer only, keep punctures to a minimum, inspect the solution and stopper, and use a new sterile needle and syringe for each transfer. You can do the same transfer using 1ml insulin syringes if your instructions allow it. For each fixed-needle transfer, draw from the diluent vial, inject into the peptide vial once, then discard. Never return that needle to the diluent vial after it has entered the peptide vial, and never move one fixed needle back and forth between vials.
  • Insulin syringe: For subcutaneous (into the fat layer just beneath the skin) doses only when specified by your prescriber, a 1ml U-100 insulin syringe may be used. Follow the prescribed route and needle size; do not use this guide for intramuscular (into muscle) injection technique.
  • Alcohol wipes: 70% isopropyl alcohol wipes for cleaning vial stoppers before every puncture.
  • Sharps container: For used needles and syringes.

3. How Much Water to Add

The amount of specified diluent you add determines the concentration of your reconstituted product, which in turn determines which visible U-100 syringe marking corresponds to your prescribed volume. This is the step where dosing errors can happen, so use the written volume from your prescriber or compounding pharmacy.

The basic formula

For these arithmetic examples, assuming the final volume is effectively the specified diluent volume added: concentration (mg/ml) = product amount (mg) divided by final or reconstituted volume (ml). If the pharmacy label gives a concentration, use that as the source of truth.

Arithmetic example only: a 5mg vial reconstituted with 2ml of the specified diluent gives a concentration of 2.5mg/ml. If the written prescribed dose were 250mcg (0.25mg), the volume would be 0.1ml, which equals 10 unit marks on a standard 1ml U-100 insulin syringe. The 250mcg number is used only to show the maths; it is not a dose recommendation.

Arithmetic examples only

Peptide Amount Specified Diluent Added Concentration Example 250mcg arithmetic =
5mg 1ml 5mg/ml 5 unit marks (0.05ml)
5mg 2ml 2.5mg/ml 10 unit marks (0.1ml)
5mg 2.5ml 2mg/ml 0.125ml (do not estimate if this is not an exact visible syringe marking)
10mg 2ml 5mg/ml 5 unit marks (0.05ml)
10mg 5ml 2mg/ml 0.125ml (do not estimate if this is not an exact visible syringe marking)

The table shows arithmetic only. The reconstitution volume must be copied from your prescriber or pharmacy instructions. Do not change the volume unless they have instructed you to do so.

These examples assume the vial is large enough for the specified volume. Do not add more diluent than the vial capacity or supplied instructions allow.

Clean vial and alcohol wipe arranged before stopper disinfection
Use a fresh alcohol wipe for each vial stopper and let the stopper dry before puncturing.

4. Step-by-Step Reconstitution

Step 1: Clean your workspace

Wash your hands thoroughly. Set out all supplies on a clean, flat surface. You want everything within reach before you start.

Step 2: Inspect the peptide vial

Before reconstituting: visually inspect the lyophilised product. Discoloration, a cracked seal, moisture, unusual appearance, or supplier-specific warning signs warrant contacting the supplier before use. Loose powder or a displaced cake can happen during handling and should be assessed against the supplied instructions.

Step 3: Clean both vial stoppers

Wipe the rubber stopper on both the specified diluent vial and the medication vial with separate alcohol wipes. Let the stoppers air-dry completely according to the wipe instructions; do not touch or blow on them after wiping.

Step 4: Draw the specified diluent

Using a transfer syringe when specified: Use the needle size specified by your pharmacy or product instructions. A 10ml 18G transfer needle can be appropriate for larger one-way transfer volumes when written instructions allow that gauge. Use fresh sterile equipment, keep punctures to a minimum, and inspect the solution and stopper. If your pharmacy instructions call for pressure equalisation, draw air equal to the volume to be withdrawn, inject it into the diluent vial, invert, and draw the target volume. Otherwise follow the supplied transfer instructions.

Using 1ml insulin syringes: Draw your target volume in one or more passes. Use a fresh insulin syringe for each transfer and discard each one before grabbing the next. Do not move a fixed needle back and forth between stock vials.

Step 5: Add water to the peptide vial (slowly)

This is the critical step. Insert the needle through the peptide vial's rubber stopper. Aim the needle at the inside wall of the vial, not directly at the powder. Slowly press the plunger to let the water trickle down the glass wall and pool at the bottom. Do not squirt the water directly onto the peptide powder.

Why this matters: Forceful addition can cause foaming and may affect some sensitive products. Gentle, wall-directed addition is a conservative technique unless the supplied instructions say otherwise.

Step 6: Let it dissolve

Do not shake the vial unless the supplied instructions specifically allow it. Shaking can create bubbles and may affect some sensitive products. Instead:

  • If the supplied instructions allow it, gently swirl or roll the vial without warming it, or
  • Let the vial sit undisturbed as directed.

The solution should match the appearance described in the product instructions. If particles remain floating after gentle swirling and the supplied instructions do not say that is expected, contact your prescriber, pharmacist, or supplier before use. Do not use a solution that is unexpectedly cloudy or contains visible particles.

Step 7: Label the vial

Write on the vial or a small adhesive label: the product name, the reconstitution date, the concentration, and the discard date from the pharmacy label or prescriber instructions. This prevents confusion, especially if you have multiple vials.

5. Calculating Your Dose After Reconstitution

For subcutaneous doses where your prescriber has specified a U-100 insulin syringe, you draw individual doses using that syringe type. The key is knowing how your syringe scale translates to your product concentration.

Your pharmacy label or prescriber/pharmacist must provide the dose volume for your exact concentration. You can use our dose volume calculator only as a cross-check; if it differs from the label or instructions, do not inject and contact your pharmacist or prescriber.

A U-100 syringe is calibrated so 100 units = 1ml and 1 unit = 0.01ml, but only measure to the visible markings on the specific syringe. For example:

  • 10 unit marks = 0.1ml
  • 25 unit marks = 0.25ml
  • 50 unit marks = 0.5ml

Example dose calculation

Example only: a 5mg prescribed vial reconstituted with 2ml of the specified diluent = 2.5mg/ml concentration.

Using an example dose of 250mcg (0.25mg) only to show the arithmetic:

Volume needed: 0.25mg divided by 2.5mg/ml = 0.1ml = 10 unit marks on the syringe

If your calculated volume is very small, ask your prescriber or pharmacist whether a smaller-capacity syringe, half-unit markings, or a different concentration is needed. U-100 insulin syringes are calibrated so 100 units = 1ml and 1 unit = 0.01ml, but you should only measure to the visible markings on the specific syringe. Do not estimate tiny or fractional volumes between visible markings.

For a detailed explanation of syringe markings and how to read them accurately, see our How to Read an Insulin Syringe guide.

6. Storage and Shelf Life

  • Storage after reconstitution: Follow the supplied storage instructions. Many products specify refrigeration at 2-8°C, but storage is product-specific.
  • Refrigeration without freezing is common for many reconstituted products, but storage is product-specific. Defer to the specific guidance that came with your product.
  • Light protection: If your product instructions say to protect from light, store it as directed, such as in its carton or another light-protected container. If refrigeration is specified, keep it refrigerated without freezing.
  • Discard date. Use only the pharmacy or prescriber's written beyond-use date. If no discard date is supplied, contact them before use; do not assume 28 days.
  • Always clean the stopper with an alcohol wipe before each withdrawal. Every needle puncture is a potential contamination event.
  • Inspect before each use. The solution should continue to match the appearance described in the product instructions. If it becomes unexpectedly cloudy, changes colour, or develops visible particles, do not use it; contact your prescriber, pharmacist, or supplier.

Storing unreconstituted peptides

Store unreconstituted vials exactly as directed by the supplier or pharmacy label. Do not freeze unless the written instructions allow it. Do not reconstitute until you are ready to begin using the product under those instructions.

7. Common Mistakes

Squirting water directly onto the powder

A direct stream can cause foaming or may affect sensitive products. Gentle wall-directed addition is a conservative technique unless the supplied instructions say otherwise.

Shaking the vial

Shaking can cause bubbles or foaming and may affect some sensitive products. Swirl gently or let the vial sit unless the supplied instructions say otherwise.

Using the wrong type of water

Tap water, distilled water, and household saline are not appropriate substitutes. Use only the sterile diluent specified by the prescriber, pharmacy, or product instructions.

Inaccurate water measurement

If you add more or less water than instructed, your concentration changes and the prescribed syringe markings may no longer apply. Contact your prescriber or pharmacist before using it. Use the smallest appropriate sterile syringe that can hold the required volume and has clear visible markings; for very small volumes, ask whether a smaller-capacity syringe is needed.

Not labelling the vial

After a week in the refrigerator, reconstituted vials can look identical. Unlabelled vials lead to confusion about concentration, reconstitution date, and which peptide is which. Label immediately after reconstituting.

Keeping reconstituted peptides too long

Even with bacteriostatic water's preservative, bacterial risk increases over time and product stability can vary. If you are not going to use the entire vial within the labelled beyond-use period, ask your prescriber or compounding pharmacy whether a smaller vial size, different concentration, or different schedule is appropriate. Do not change the reconstitution volume unless instructed.

When to call your prescriber

A small drop of blood or a brief sting can occur with injections. The list below covers things that warrant a phone call to your prescriber:

  • Redness, warmth, swelling, spreading tenderness, or pus at the injection site, especially if it worsens or does not settle
  • A hard lump that doesn't go away after a week, or one that's hot and tender
  • Fever, chills or feeling unwell after an injection
  • Any rash, hives or itching at or beyond the injection site (possible allergic reaction)
  • Signs of a severe allergic reaction - swelling of face or throat, difficulty breathing - call 000 immediately
  • You realise you injected from a vial that was stored outside the temperature or time limits in the product instructions, or had unexpected cloudiness, discoloration, particles, or damage

Frequently Asked Questions

What is the difference between bacteriostatic water and sterile water?

Bacteriostatic water contains 0.9% benzyl alcohol as a preservative that helps inhibit bacterial growth. Sterile water contains no preservative. Bacteriostatic water may be specified for some multi-dose products, but diluent choice and beyond-use date are product-specific. Follow the prescriber, pharmacy, or manufacturer instructions.

How much bacteriostatic water do I add to a 5mg peptide vial?

There is no single correct answer because it depends on the specified concentration, dose, diluent, and product instructions. Your prescriber or compounding pharmacy should provide the volume in writing. Do not use a default volume if instructions are missing.

Can I use bacteriostatic water for semaglutide?

Semaglutide is a prescription medicine, and most injectable GLP-1 products are supplied ready to inject. Australian note: TGA guidance states that GLP-1 receptor agonist analogues are generally not covered by compounding exemptions, and TGA material on the 2024 changes says compounding GLP-1 receptor agonists was no longer permitted from 1 October 2024. Do not add water or use lyophilised powder unless the product and written instructions are supplied through a lawful prescriber or pharmacy pathway. Confirm the lawful product instructions with your prescriber or pharmacist before using any vial-supplied GLP-1 product.

What syringe do I use to add the bacteriostatic water?

Use the sterile transfer equipment specified by your pharmacy or product instructions. If those instructions specifically allow a 10mL syringe with an 18G draw/transfer needle for an appropriate larger-volume transfer, it can move larger volumes in one pass. Use it for draw/transfer only, keep punctures to a minimum, and inspect the stopper and solution. If you use insulin syringes instead, use a fresh one for each transfer. Do not re-enter the diluent vial with a needle that has already entered the peptide or medication vial, and do not reuse a needle or syringe for another transfer or withdrawal.

My peptide did not dissolve completely. Is it still good?

Give it time. Some products may take longer to dissolve. Follow the time and handling instructions supplied with the product; if none are supplied and particles remain, contact the prescriber, pharmacist, or supplier before use. If particles remain after the time specified in the instructions, do not use the vial; contact your prescriber, pharmacist, or supplier.

Can I store reconstituted peptides in the freezer?

Storage after reconstitution is product-specific. Many supplied instructions call for refrigeration without freezing, but you should defer to the specific guidance that came with your product.

Tip: a vial storage case keeps your vials upright, cushioned and organised. When travelling, maintain the product's written temperature and light-protection requirements, and ask your pharmacist about allowed temperature excursions or cold-chain storage.

Supplies for peptide reconstitution and injection: