Reference

Peptide Reconstitution Glossary

Plain-English definitions of the core terms used across BAC water and peptide reconstitution. Whether you are reading a guide, following a protocol, or working through the math, this reference explains what words like bacteriostatic water, diluent, concentration, and U-100 actually mean. It is for research and educational use, not medical advice.

Compiled and maintained by the BACwater.ai editorial team and checked against the sources cited on this page. This is general research information, not a medical review. Last updated July 2026.

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Bacteriostatic water
Benzyl alcohol
Bacteriostatic
Reconstitution
Lyophilization / lyophilized
Diluent
Concentration (mg/mL)
U-100
Subcutaneous
Sterile water

Bacteriostatic water

This is sterile water with 0.9% benzyl alcohol (a germ-fighting preservative) added. The preservative stops germs from growing. Because of it, you can put a needle into the same vial many times. That is why it is used to dissolve and mix dried peptides. It is often called BAC water for short. Per FDA labeling, it is not for use in newborns.

Learn more: What is BAC water?

Benzyl alcohol

This is the germ-fighting preservative in BAC water. It makes up 0.9% of the water by weight. It slows germ growth, so you can use one vial across many needle draws. Benzyl alcohol is what makes BAC water different from plain sterile water.

Learn more: Benzyl alcohol, explained

Bacteriostatic

"Bacteriostatic" means it stops germs from growing. It does not kill germs that are already there. (A different word, "bactericidal," means it kills germs.) The preservative in BAC water is bacteriostatic. So the water holds off new germs, but it does not kill germs that already got in.

Reconstitution

This means adding a liquid to a dried powder to turn it back into a usable liquid. The liquid you add is called a diluent. For peptides, this usually means adding BAC water to a vial of dried powder. How much you add sets how strong the final liquid is.

Learn more: How reconstitution works

Lyophilization / lyophilized

Lyophilization is just freeze-drying. First the liquid is frozen. Then the water is pulled out, leaving a dry powder or cake. A lyophilized peptide is one that comes in this dried form. Freeze-drying helps the peptide stay stable during shipping and storage until you mix it.

Diluent

A diluent is the liquid you use to dissolve or thin a dried substance when you mix it. For peptides, BAC water is the most common diluent. Its preservative lets you draw from the vial many times. Sterile water and other liquids can also be diluents, based on the product.

Concentration (mg/mL)

This is how much peptide is in each milliliter of liquid. It is shown as milligrams per milliliter (mg/mL). To find it, divide the total milligrams of peptide by the amount of liquid you added. More liquid makes it weaker. Less liquid makes it stronger.

Learn more: Dose calculator

U-100

This is a standard for insulin syringes. The scale is marked in units, and 100 units equal 1 milliliter. So on a U-100 syringe, 1 unit is 0.01 mL. Peptide users borrow these syringes and their unit marks to measure small amounts with care.

Learn more: Syringe units explained

Subcutaneous

"Subcutaneous" means the layer of fatty tissue just under the skin. A subcutaneous injection puts fluid into that layer. It usually uses a short, thin needle. It is often written as subQ or SC for short.

Sterile water

This is sterile water for injection. Sterile means it had no germs in it when it was made. But it has no preservative. Without one, an opened vial is for a single use only. That is the main difference from BAC water, which you can draw from more than once because of its preservative.

Learn more: Sterile water vs BAC water

Common questions

Both are sterile at the point of manufacture, but bacteriostatic water contains 0.9% benzyl alcohol as a preservative while sterile water contains no preservative. The preservative lets a bacteriostatic vial be punctured and re-entered multiple times, whereas a sterile water vial is intended for single use once opened.

U-100 means the syringe is scaled so that 100 units equal 1 milliliter, so each unit is 0.01 mL. The unit markings make it easier to measure the very small volumes common in peptide reconstitution without converting to fractions of a milliliter.

References

Primary sources for the facts on this page. We cite regulatory and peer-reviewed authorities rather than secondary blogs.

Bacteriostatic Water for Injection, USP - prescribing information · U.S. FDA labeling via DailyMed (NIH / NLM)Defines bacteriostatic water as sterile water with 0.9% benzyl alcohol added as a bacteriostatic preservative, supplied in a multiple-dose container for diluting or dissolving drugs; contraindicated in neonates.
Benzyl alcohol (Compound CID 244) · NIH PubChem, National Library of MedicineChemical identity, properties, and safety data for benzyl alcohol, the bacteriostatic preservative in BAC water.

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