How to Reconstitute Peptides: The Math (and Mistakes) That Actually Matter

Get the ratio wrong here and every dose that follows is wrong too — quietly, since nothing about a cloudy-free, correctly-dissolved vial tells you the concentration inside it is off. Reconstitution isn't hard, but it's exactly the kind of step where a small arithmetic mistake compounds silently.

The Core Math

Three numbers, one formula:

mg in the vial ÷ mL of bacteriostatic water added = mg/mL concentration.

A 5mg vial mixed with 2mL of bacteriostatic water gives 2.5mg/mL — or 2,500mcg/mL. On a standard 1mL (100-unit) insulin syringe, that's 25mcg per unit. Add 5mL instead of 2mL and the same vial becomes 1mg/mL (10mcg/unit) — same peptide, same vial, a completely different number on the syringe for the same intended dose.

There's no universal "correct" amount of water to add — it's a choice that trades off concentration against how many units you have to draw for a given dose. More water means a more forgiving, easier-to-read draw for small doses; less water means fewer injections' worth of volume per vial. What matters is doing the division correctly once you've picked an amount, and sticking to that same ratio every time you refill the same syringe type.

Bacteriostatic Water Isn't Just "Water"

Bacteriostatic water is sterile water with a small amount (typically 0.9%) of benzyl alcohol added as a preservative — that's the whole reason it exists as a separate product from plain sterile water. The preservative is what makes it safe to draw from the same vial multiple times over its usable life instead of needing a fresh sterile vial for every single draw. Plain sterile water has no such preservative — once it's opened, it doesn't stay clean for repeated use the way bacteriostatic water does.

Reading the Syringe

Insulin syringes are marked in units (IU), not milliliters — a standard "100-unit" syringe holds 1mL, meaning 1 unit = 0.01mL. That's the scale the mcg-per-unit math above is built on. Two things trip people up here:

• Dead space. The small gap between the needle and the plunger holds a bit of liquid that never gets drawn into the barrel's marked scale. It's usually negligible at normal draw volumes, but becomes a real proportional error on very small draws — worth knowing, not worth panicking over.
• Bubbles. An air bubble in the barrel reads as volume on the scale without actually being liquid — always tap and expel air before treating the reading as accurate.

After It's Mixed

Once reconstituted, keep the vial refrigerated and out of direct light — heat and light are what degrade a dissolved peptide faster than time alone does. Don't freeze it after mixing. If the liquid turns cloudy, changes color, or you're not sure how long it's been sitting, don't use it — a peptide that looked fine last week isn't worth guessing about now.

How long a reconstituted vial stays good for varies by peptide and isn't something we verify site-wide, so treat any specific shelf-life number as a starting point to confirm against the actual product you bought, not a fact to assume.

The Mistakes That Actually Cost People

Not the exotic ones — the boring ones:

• Doing the math in your head mid-draw instead of writing down mg/mL once and reusing it. This is where the silent errors above come from.
• Reusing sterile (non-bacteriostatic) water across multiple draws. Without the preservative, that's an open door for contamination the second vial gets used more than once.
• Losing track of which vial has which concentration when more than one is reconstituted at different ratios — label them.

None of this is complicated once it's written down once. It's forgettable precisely because it's simple — which is exactly why it's worth having somewhere to check rather than trusting memory every time.