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How to Reconstitute Peptides: Step by Step
Most peptides arrive as a freeze-dried (lyophilized) powder. Reconstitution means dissolving that powder in sterile liquid so the dose can be measured. The method is simple, but a few details decide whether the peptide stays intact.
What you need
- The sealed peptide vial and a sterile diluent, normally bacteriostatic water (sterile water with 0.9% benzyl alcohol).
- Sterile syringes: one for mixing and insulin syringes for dosing.
- Alcohol swabs for the vial stoppers.
- A clean, flat surface, and a label or marker for the date.
Bacteriostatic water or sterile water?
Bacteriostatic water contains a preservative that discourages bacterial growth, so a vial can be punctured several times over a few weeks. Sterile water has no preservative and is meant for a single use; once the vial is opened it should be used promptly.
For a multi-dose vial, bacteriostatic water is the usual choice. Some compounds, and some people, do not tolerate benzyl alcohol, so check the compound notes first.
Step by step
- Let the vial warm to room temperature if it has been in the fridge, and wash your hands.
- Swab the rubber stoppers of the peptide vial and the water vial and let them dry.
- Draw the chosen volume of water into the syringe. Remove large air bubbles.
- Insert the needle through the peptide stopper at an angle so the stream runs down the inside wall of the vial.
- Push the plunger slowly. Dropping water straight onto the powder can damage fragile peptides.
- Withdraw the needle and let the vial sit for a minute. Then swirl very gently or roll it between your fingers until the liquid is clear. Do not shake.
- Write the date on the vial and refrigerate it at 2–8 °C, away from light.
If the solution stays cloudy, has particles, or changes colour, do not use it.
Choosing how much water to add
The amount of water does not change how much peptide is in the vial. It only changes the concentration, which changes how many units you draw. More water means a lower concentration and a larger, easier-to-measure volume per dose. Less water means a stronger solution and a smaller volume.
A practical target is a dose that lands between roughly 10 and 50 units on a standard insulin syringe. Below about 5 units the marks are too close together to read accurately; above 100 units the dose will not fit in a 1 mL syringe.
The formula
Concentration (mg per mL) = vial amount (mg) ÷ water added (mL).
Volume to draw (mL) = dose (mg) ÷ concentration (mg per mL).
On a U-100 insulin syringe, every 0.01 mL is one unit, so units = volume in mL × 100.
Worked examples
Check the first row by hand. 5 mg ÷ 2 mL = 2.5 mg/mL. A 250 mcg dose is 0.25 mg, and 0.25 ÷ 2.5 = 0.1 mL. On a U-100 syringe, 0.1 mL is 10 units.
The same vial also tells you how many doses it holds: 5 mg is 5,000 mcg, and 5,000 ÷ 250 = 20 doses.
| Vial | Water | Concentration | Dose | Draw | Syringe units |
|---|---|---|---|---|---|
| 5 mg | 2 mL | 2.5 mg/mL | 250 mcg | 0.10 mL | 10 |
| 10 mg | 3 mL | 3.33 mg/mL | 500 mcg | 0.15 mL | 15 |
| 2 mg | 1 mL | 2 mg/mL | 200 mcg | 0.10 mL | 10 |
| 30 mg | 3 mL | 10 mg/mL | 2.5 mg | 0.25 mL | 25 |
Common mistakes
- Mixing up mg and mcg. A thousand-fold error is the most dangerous mistake in this field.
- Assuming one unit on the syringe equals one IU of the hormone. Syringe units measure volume; IU measures biological activity.
- Shaking the vial hard, which can fragment the peptide and make foam.
- Using a vial size that differs from the protocol without recalculating the water volume.
- Leaving a reconstituted vial at room temperature for days.
Common questions
How long does a reconstituted peptide last?
Many are kept in the refrigerator for about two to four weeks, but it varies by compound. Check the storage notes on its page and discard anything cloudy.
Can I add more water than the protocol says?
Yes, as long as you recalculate. The peptide amount stays the same; only the volume per dose changes.
Why does the powder look like a tiny clump or film?
Freeze-drying leaves a small, loose cake, and sometimes only a thin film on the glass. That is normal.
Do I need to warm bacteriostatic water first?
No. Cold water works, but letting both vials reach room temperature makes the steps more comfortable.
Try it
- Dosage Calculator: Vial size, water and dose in; mL and syringe units out, plus doses per vial.
- Syringe Simulator: See the plunger line on a syringe, then test yourself with practice problems.