To reconstitute a peptide you add a measured volume of sterile or bacteriostatic water to the lyophilised powder, and the amount of water you add sets the concentration, not the dose. A 5 mg vial mixed with 2 mL of water gives 2.5 mg per mL. Mixed with 1 mL it gives 5 mg per mL. Same vial, same peptide, double the strength per unit on the syringe.
That is the whole idea, and almost every mistake in this category comes from one of three numbers: the milligrams in the vial, the millilitres of water you add, and the units printed on the syringe you draw with. Get those three right and the rest is technique. If you are reconstituting a prescribed medicine, follow the label and your clinician’s instructions exactly; this page explains the maths and the handling, not what to take.
In This Guide
- The only formula you need
- Reconstitution and dosing chart
- Reading the syringe: units, millilitres and the U-40 trap
- How a regulator does the same maths
- How to reconstitute a peptide, step by step
- Six mistakes that ruin a vial
- What the labels say about storage after mixing
- FAQ
The only formula you need
Milligrams in the vial, divided by millilitres of water, gives the concentration in mg per mL. Everything else follows from that number.
To turn concentration into something you can read on a syringe, multiply by 1,000 to get micrograms per mL, then divide by 100, because a U-100 syringe divides each millilitre into 100 units. One unit equals 0.01 mL.
So for a 5 mg vial mixed with 2 mL of water: 5 รท 2 = 2.5 mg per mL, which is 2,500 micrograms per mL, which is 25 micrograms per unit. Ten units on the syringe is 250 micrograms.
Reconstitution and dosing chart
The chart below covers the vial sizes and water volumes people actually use. Read it as: this is what a given combination produces, so you can pick the volume rather than guess it.
| Vial size | Water added | Concentration | Per unit on a U-100 syringe | 10 units draws |
|---|---|---|---|---|
| 2 mg | 1 mL | 2 mg/mL | 20 mcg | 200 mcg |
| 2 mg | 2 mL | 1 mg/mL | 10 mcg | 100 mcg |
| 2 mg | 3 mL | 0.67 mg/mL | 6.7 mcg | 67 mcg |
| 5 mg | 1 mL | 5 mg/mL | 50 mcg | 500 mcg |
| 5 mg | 2 mL | 2.5 mg/mL | 25 mcg | 250 mcg |
| 5 mg | 3 mL | 1.67 mg/mL | 16.7 mcg | 167 mcg |
| 10 mg | 1 mL | 10 mg/mL | 100 mcg | 1,000 mcg |
| 10 mg | 2 mL | 5 mg/mL | 50 mcg | 500 mcg |
| 10 mg | 3 mL | 3.33 mg/mL | 33.3 mcg | 333 mcg |
Figures assume a U-100 syringe, where 100 units equal 1 mL. They describe concentration only, not what anyone should take.
Two things worth noticing in that table. More water makes a weaker solution, which means a larger volume to draw for the same amount of peptide, and larger volumes are harder to measure accurately on a small syringe. Less water makes a stronger solution, which makes small differences in the draw matter more. The useful middle is usually enough water that the volume you draw lands between 5 and 50 units, where the markings are easy to read.
Reading the syringe: units, millilitres and the U-40 trap
A U-100 insulin syringe divides 1 mL into 100 units, so 100 units is 1 mL and every single unit mark is 0.01 mL. This is the standard in the United States, the United Kingdom and most of the EU, and it is why the chart above can talk in units at all.
The trap is that U-40 syringes also exist, in some markets and in veterinary supply, where the scale divides 1 mL into 40 units. The same physical volume is a different number of units on a U-40 syringe, so a dose read in units from a page like this one, then drawn on a U-40 barrel, is wrong by a factor of two and a half. Before drawing anything, check the barrel: the scale is printed on it, and the packaging states U-100, U-40, or units per mL. When in doubt, work in millilitres, since 1 mL is 1 mL on every syringe in the world.
How a regulator does the same maths
This is not a grey-area calculation invented by forum users. Regulators publish reconstitution instructions in exactly this form, which makes them a good check on your own arithmetic.
- Tesamorelin (Egrifta WR): the label directs that an 11.6 mg lyophilised vial be reconstituted with 1.3 mL of bacteriostatic water, and states the resulting concentration as about 8 mg per mL, with the 1.28 mg dose drawn as 0.16 mL. Check the last part: 8 multiplied by 0.16 is 1.28, so the label is internally consistent. Note the first part, though. Dividing 11.6 by 1.3 gives 8.9, not 8, and that gap is the point: vials are overfilled, and some of the solution stays in the vial and the needle. The published concentration accounts for that, which is exactly why manufacturers state a concentration instead of leaving you to divide. Use the published figure where there is one, and divide when there is not.
- Tesamorelin (Egrifta SV): the earlier formulation reconstitutes with 0.5 mL of diluent to give 2 mg per 0.5 mL, and the label specifies rolling the vial gently in your hands for 30 seconds to mix.
- Somatropin products: growth hormone labels take the same approach, with a fixed diluent volume per vial and a stated concentration afterwards.
Two details in those labels are worth copying into your own routine: the volume is chosen by the manufacturer so the dose lands on a readable part of the syringe, and mixing is done by rolling or swirling, never by shaking.
How to reconstitute a peptide, step by step
- Read the vial and know the target dose. The milligram figure on the label is the number that everything else divides into. If a vial says 5 mg, that is 5 mg of peptide in the vial, regardless of how much it looks like.
- Choose the water volume and write down the concentration before you start. Decide, then do the arithmetic from the chart above and note the result. Deciding afterwards is how people end up unsure what they have in the vial.
- Clean the surface, your hands, and both vial tops. Alcohol swab on the rubber stoppers, then let them dry. A wet stopper carries the alcohol into the vial.
- Draw the water. Pull air into the syringe equal to the volume you need, inject that air into the water vial, invert it, and draw the water. The air first makes the draw easier.
- Add air to the powder vial, then add the water. Injecting a little air first reduces the pressure difference. Then push the plunger slowly.
- Aim at the glass wall, not the powder. Let the water run down the inside of the vial rather than blasting straight into the cake, which is the main cause of clumping and foaming.
- Swirl or roll to mix. Never shake. Peptides are fragile, and shaking creates foam and shear forces. Most vials dissolve within a minute or two of gentle rolling.
- Label it, store it, and use a new needle and syringe every time. Write the concentration and the date on the vial. Reusing a needle or syringe is the one shortcut with a real infection risk attached to it.
Six mistakes that ruin a vial
| Mistake | What actually happens | What to do instead |
|---|---|---|
| Shaking the vial | Foam, shear forces, and in some cases aggregated or damaged peptide that never fully dissolves | Roll or swirl; give it a minute |
| Blasting water at the powder cake | The powder clumps into a plug that takes far longer to dissolve, or never does | Aim at the glass wall and let it run down |
| Adding too much water | A weak solution that needs a large draw, which is harder to measure and less comfortable to inject | Choose a volume that puts the draw between 5 and 50 units |
| Using the wrong syringe scale | Units read from a U-100 chart and drawn on a U-40 barrel, or the reverse, are wrong by a factor of 2.5 | Check the barrel marking; work in mL when unsure |
| Reusing needles or sharing vials | Contamination and infection. This is the exact practice that health agencies spend the most effort warning against | New needle and syringe every time; single-use vials stay single use |
| Freezing after mixing | Ice crystal formation damages peptide structure, and labels explicitly say not to freeze reconstituted product | Refrigerate, protect from light |
What the labels say about storage after mixing
Two details decide how long a mixed vial is good for: whether the diluent contained a preservative, and what the manufacturer validated.
- With bacteriostatic water (which contains benzyl alcohol), one somatropin label allows refrigeration at 2 to 8 degrees Celsius for up to 14 days after reconstitution.
- With the manufacturer’s own diluent, another somatropin product states that the mixed solution must be used within 28 days.
- Never freeze a reconstituted vial. Refrigeration is the instruction; the freezer is not.
- Protect from light, and keep the vial upright once mixed.
The difference between 14 and 28 days is not a rounding error, and it comes from the product and the diluent rather than from a general rule. The full breakdown of storage windows, temperatures and the signs a solution has gone off is in our peptide half-life chart and the guides collected in the peptides section. If you are mixing a prescribed product, its own label overrides everything on this page.
FAQ
How much water do I add to a peptide vial?
There is no single correct volume, because the water sets the concentration rather than the dose. Choose a volume that makes the amount you need land between about 5 and 50 units on a U-100 syringe, then write the resulting concentration on the vial. Where a manufacturer specifies a volume, as tesamorelin labels do, use theirs.
What does 10 units mean on a syringe?
On a U-100 syringe, 10 units is 0.1 mL, because 100 units equal 1 mL. What that volume contains depends entirely on the concentration: at 2.5 mg/mL it is 250 micrograms, while at 5 mg/mL it is 500 micrograms.
Can I use tap water or bottled water?
No. Reconstitution requires sterile water for injection or bacteriostatic water for injection. Tap and bottled water are not sterile and are not intended for injection.
What is the difference between bacteriostatic water and sterile water?
Bacteriostatic water contains benzyl alcohol, a preservative that discourages bacterial growth and is what allows a vial to be drawn from more than once. Sterile water for injection contains no preservative at all, so it is intended for single use. Preservative-free diluent usually means a shorter usable window after mixing.
How do I mix it without damaging the peptide?
Let the water run down the inside wall of the vial rather than aiming at the powder, then roll the vial between your hands or swirl it gently. Never shake it, and never stir it with the needle.
How long does a reconstituted peptide last?
It depends on the product and the diluent. Published labels for reconstituted growth hormone products range from 14 days when mixed with bacteriostatic water to 28 days with the manufacturer’s diluent, refrigerated and never frozen. Treat your own vial’s label as the authority.
Can I use a U-40 syringe instead?
Only if you convert the volume rather than the units. The same physical volume is a different number of units on a U-40 barrel, so a dose in units read from a U-100 chart will be wrong. Working in millilitres avoids the problem entirely.
Does the powder need to be at room temperature first?
Let a refrigerated vial sit for a few minutes so it reaches room temperature before you add water. It dissolves more evenly, and it avoids condensation on the stopper.
The bottom line
Reconstitution is arithmetic plus clean technique. Milligrams divided by millilitres gives the concentration; a U-100 syringe gives you 100 units per millilitre; and the volume you choose decides how easy the draw is to measure accurately.
Write the concentration on the vial the moment you mix it, check the scale printed on your syringe barrel, and treat the manufacturer’s label as the final word on volumes and storage windows. Everything else on this page is there to make those three habits easy to follow.
Last reviewed: October 2026 by Julian Hart. This page is educational and is not medical advice, and it is not a set of instructions for using any substance. If you are reconstituting a prescribed medicine, follow the label and your clinician’s directions, use a new needle and syringe for every injection, and dispose of sharps properly.
Sources
- DailyMed (NIH). Egrifta WR (tesamorelin) label: 11.6 mg vial reconstituted with 1.3 mL bacteriostatic water, dose 1.28 mg in 0.16 mL.
- DailyMed (NIH). Egrifta SV (tesamorelin) label: 0.5 mL diluent for a 1 mg vial, mixed by rolling gently for 30 seconds.
- DailyMed (NIH). Bacteriostatic water for injection label, including the benzyl alcohol preservative.
- DailyMed (NIH). Sterile water for injection label: no bacteriostat or antimicrobial agent.
- MedlinePlus (NIH). Giving an insulin injection: each mark on a standard 1 mL insulin syringe is one unit.
- MedlinePlus (NIH). Drawing medicine out of a vial.
- CDC. Preventing unsafe injection practices: one needle and one syringe per injection, single-dose vials used once.
- Serostim (somatropin) prescribing information: reconstituted with bacteriostatic water, refrigerate at 2 to 8 degrees Celsius for up to 14 days.
- Humatrope (somatropin) labeling: mixed product must be used within 28 days.
