Bacteriostatic vs Sterile Water, and How to Reconstitute

📅 July 13, 2026 ⏱️ 3 min read ✍️ Team TPS

Bacteriostatic water and sterile water are not the same thing, and using the wrong one is the difference between a peptide that keeps for weeks and one that keeps for a day.

The difference in one line

Sterile water contains nothing but water. It is sterile when sealed, and stops being sterile the moment a needle enters it.

Bacteriostatic water contains 0.9% benzyl alcohol, which inhibits bacterial growth. That allows a vial to be entered repeatedly and remain usable, typically around 28 days refrigerated.

For anything you will draw from more than once, bacteriostatic is the correct choice. Sterile water is single-use by nature.

Why the volume you choose changes everything downstream

Reconstitution is where most dosing confusion begins, because the volume is arbitrary and you pick it.

Ten milligrams of peptide in 1ml is ten times the concentration of the same 10mg in 10ml. The peptide is identical; the amount per unit on your syringe is not. Every subsequent calculation depends on a number you chose at the start, which is precisely the problem a pre-filled pen removes.

Write the volume on the vial. Not mentally, on the vial, in pen.

How to actually mix it

Let it come to temperature. A cold vial straight from the fridge invites condensation.

Wipe both stoppers with alcohol and let them dry.

Aim the water at the glass wall, not directly onto the powder. Let it run down.

Do not shake. This is the one people get wrong. Peptides are fragile and agitation damages them. Swirl gently, or leave it, most peptides dissolve on their own within minutes.

Inspect. Clear solution, no particles. Cloudiness means stop.

The clock starts now

Dry, lyophilised peptide is stable for a long time. In solution it is not.

Once reconstituted you are on a refrigerated countdown measured in weeks, and heat, light and agitation all shorten it. That is why reconstituting more than you will use is a false economy. See storing peptides properly.

Why pens sidestep all of this

A pre-filled pen arrives already in solution at a fixed concentration, sealed, with no mixing step and no volume decision. The in-use clock does not start until you attach a tip.

The trade is that you cannot choose the concentration. For most people that is a benefit, not a limitation. See how to read a peptide pen dial and your first week with a peptide pen.

Supplied for research purposes only.