How to Reconstitute GLP-1 Peptides — The Mixing Math, Made Simple

Reconstituting a peptide vial trips people up for one reason: a single vial gets described three different ways. The label says milligrams, doses are written in micrograms, and the syringe is marked in units. Once you see how those three connect, mixing a vial becomes simple arithmetic.

This guide covers what reconstitution is, the one formula that ties everything together, and a worked example you can copy. It's educational only — how much of any compound to take, and whether to take it at all, is a decision for you and a qualified healthcare provider. Here we're only talking about the mixing math.

Why a vial has to be reconstituted

Most peptide vials arrive lyophilized — freeze-dried into a small puck or film at the bottom of the vial. In that state it can't be measured or drawn. You make it usable by adding bacteriostatic water (sterile water with 0.9% benzyl alcohol, which keeps the mixed vial stable for weeks). This step is reconstitution.

The single most important idea: how much water you add does not change how much peptide is in the vial. A 5 mg vial holds 5 mg whether you dissolve it in 1 mL or 3 mL. What the water changes is the concentration — and therefore how many units you draw for a given amount. More water = more units per draw (easier to measure small amounts); less water = fewer units.

mg, mcg, and units — the three numbers

• Milligrams (mg) — how much peptide is in the vial, printed on the label (e.g. 5 mg).
• Micrograms (mcg) — how amounts are usually written. 1 mg = 1,000 mcg, so 0.25 mg = 250 mcg.
• Units (u) — the marks on an insulin syringe. A U-100 syringe has 100 units per 1 mL, so each unit is 0.01 mL. Units are a volume, not a fixed amount of peptide — how much peptide sits in each unit depends entirely on your concentration.

The whole job of reconstitution math is turning "I want 250 mcg" into "draw to the ___ line."

The formula (and a worked example)

The relationship is:

units to draw = amount (mcg) ÷ mcg per unit, where mcg per unit = (vial mg × 1,000) ÷ (water mL × 100)

Say you have a 5 mg vial and add 2 mL of bacteriostatic water:

1. Peptide in vial: 5 mg × 1,000 = 5,000 mcg
2. Concentration: 5,000 mcg ÷ 2 mL = 2,500 mcg/mL
3. Per unit: 2,500 ÷ 100 = 25 mcg per unit
4. For a 250 mcg amount: 250 ÷ 25 = 10 units

So 250 mcg lands on a clean 10 units on a U-100 syringe — easy to read and repeat. Change nothing about the mix and 500 mcg is simply 20 units, because you're drawing from the same concentration.

Rather than do this by hand each time, drop your numbers into the peptide calculator — it shows the exact units to draw, the concentration, and how many doses the vial holds, plus a syringe diagram of where to fill to. If you'd rather pick a round number of units first, it can work backward and tell you how much water to add to hit it.

Choosing how much water to add

Because the water only sets your units-per-draw, you can pick it to make measuring easy:

• More water spreads a small amount across more units — good when you'd otherwise be drawing 2–3 units, which are hard to measure precisely.
• Less water concentrates it into fewer units — fine when the amount is large.
• Aim for a whole number of units. If your draw lands between the printed marks (say 9 units on a barrel marked every 2), nudging the water slightly can make every draw land on a clean line. The calculator's "help me pick water" mode does this for you.

A quick word on storage

Once mixed, a vial is a liquid with a limited life:

• Refrigerate it (about 2–8 °C / 36–46 °F); reconstituted with bacteriostatic water it's commonly considered stable for up to ~28 days.
• Don't freeze the reconstituted vial — freezing can degrade the peptide.
• Keep it out of light and wipe the stopper with alcohol before each draw.
• Write the mix date on the vial so you know when the ~4-week window is up.

(We'll cover storage and shelf life in depth in a later article.)

Common mistakes to avoid

• Confusing mg and mcg. Entering "5 mg" where you meant "5 mcg" is a 1,000× miss — the most common error there is. If a draw looks like it would use up the whole vial at once, re-check your units.
• Assuming more water = stronger. It's the opposite intuition trap. Water changes units-per-draw, never the amount of peptide.
• Drawing between the marks. Hard to measure accurately; adjust the water so draws land on a printed line.
• Losing track of the ~28-day window. Date the vial; don't guess.

Frequently asked questions

Does the amount of bacteriostatic water change anything about the peptide? Only the concentration, and therefore how many units you draw. The amount of peptide in the vial is fixed.

What syringe should I use? A U-100 insulin syringe. The 0.3 mL (30-unit) and 0.5 mL (50-unit) barrels are marked every 1 unit, which makes small draws easier to read than the 1 mL (100-unit) barrel, which is marked every 2 units.

How many draws will a vial give me? Divide the total vial amount by your per-draw amount. A 5 mg (5,000 mcg) vial at 250 mcg is 20 draws. The calculator shows this as "doses per vial" automatically.

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Reconstitution feels intimidating the first time and trivial the fifth. Hold onto one idea and you're most of the way there: the water sets your units, not the amount of peptide. Everything else is multiplying and dividing by 1,000 and 100 — and the calculator does that for you, then shows exactly where to fill.