BAC water vs sterile water vs acetic acid: which to use

For most peptides, the usual choice is bacteriostatic (BAC) water, because the preservative lets you use the same vial for several doses. Sterile water has no preservative and is generally treated as single-use, while dilute acetic acid is used for a small group of peptides that dissolve poorly in plain water.

This is general harm-reduction information. Many of these compounds are unlicensed research chemicals, and what follows is not medical advice or a recommendation to use anything.

The quick comparison

• Bacteriostatic water: sterile water with 0.9% benzyl alcohol as a preservative. Multi-dose friendly. The default for most peptides.
• Sterile water (water for injection): sterile, no preservative. Safe from contamination only until it is first opened, so it is generally treated as single-use.
• Dilute acetic acid water: sterile water with a very small amount of acetic acid (the acid in vinegar), commonly around 0.6%, though strengths vary, so read the label. Used for peptides that stick together or dissolve poorly at neutral pH.

Bacteriostatic water

"Bacteriostatic" means it inhibits the growth of bacteria rather than killing them outright. The preservative is benzyl alcohol, at roughly 0.9%. That does not make the water magically safe if you are sloppy, but it does reduce the chance that a bug you accidentally introduce on one puncture multiplies before the next. See the bacteriostatic water page for more.

Why it is the usual pick

• A vial of peptide is typically used over days to weeks, so a preservative is useful
• It is the standard diluent referred to on pages such as BPC-157 and TB-500
• It is widely available

Drawbacks

• Benzyl alcohol is not suitable for everyone. It is not recommended for newborns, and some people find it stings or irritates
• A few people and a few peptides do poorly with it, in which case plain sterile water is chosen instead
• Once opened, a BAC water vial is usually discarded after about 28 days, which is common labelling guidance for multi-dose vials. Check the label on yours

Sterile water

Sterile water for injection is clean but unpreserved. Because there is nothing to hold back bacterial growth, an opened vial should be treated as a single-use item and not kept for later. Some people use it to mix a peptide they plan to use all at once, or where benzyl alcohol is a problem.

The downside is shelf life. A peptide mixed with sterile water has no built-in protection, so many users keep it for a shorter time than the same peptide mixed with BAC water, and some use it within a day or so. There are no tidy official numbers for most research peptides, so be conservative.

Acetic acid water

A small number of peptides are poorly soluble at neutral pH and clump or cloud in plain water. IGF-1 LR3 is the classic example, and it is commonly described as being dissolved in dilute acetic acid. The slightly acidic environment helps keep the peptide dissolved and can help stability. See acetic acid water for specifics.

Things to know

• Concentration matters. It is dilute, and a stronger solution than intended can sting and damage tissue
• It usually comes pre-made and sterile from a supplier. Making it yourself from kitchen vinegar is not sterile and is a bad idea
• It is for specific peptides. Using it where it is not needed adds a variable for no benefit

How to choose

1. Check the peptide's own page for the diluent usually recommended. For most, that is BAC water.
2. If it is a stubborn peptide that stays cloudy or clumps in BAC water, acetic acid water is the usual alternative. Do not just keep adding more liquid.
3. If you cannot tolerate benzyl alcohol or the peptide will be used in one go, sterile water is the fallback.
4. If you are unsure, do not guess. Ask the supplier for the product's recommended diluent, and treat vague answers as a red flag.

Practical tips for any diluent

• Check the vial is sealed, in date and labelled sterile before you buy or use it. A loose cap or missing label is a reason to bin it
• Write the date you first punctured any multi-dose water vial on the label
• Use a fresh needle every time you draw from it, and swab the stopper first
• Do not share a water vial between people
• Do not mix and match: if a peptide has been dissolved in acetic acid water, do not then add BAC water on top to "top it up"
• Keep diluent vials in a cool, clean place. Most are stored at room temperature unless the label says otherwise

The best evidence for any of these choices comes from the product labelling and the supplier's guidance for that specific peptide, which is why those details are worth keeping. Unlicensed products often come with thin or inconsistent instructions, so be sceptical where the information is missing.

Does the water change the dose maths?

No. The water just carries the peptide. The mg in the vial and the ml of liquid you add decide the concentration, regardless of which sterile liquid you use. Our reconstitution guide walks through the sums, and the dosing maths article covers common mistakes.

Does the water change how long it lasts?

Yes, in practice. A preserved, multi-dose-friendly liquid generally gives you more margin than unpreserved water. But shelf life also depends on the peptide itself, how cold you keep it and how clean your technique is. See how to store peptides.

Bottom line

Use BAC water unless the peptide or your own situation says otherwise. Use sterile water for short-term, single-use mixing, and use acetic acid only for peptides known to need it. Whatever you choose, it must be sterile, from a sealed and labelled source, and it is your job to check that. These are mostly unlicensed compounds, so there is no regulator checking the process for you.