How to reconstitute a peptide vial

A peptide vial arrives as a small amount of white powder, sometimes so little that it looks like the vial is empty. Reconstitution is the step that turns that powder into a liquid that can be measured. It is a mechanical procedure with a handful of ways to get it wrong, and most of them are invisible: a vial that has been mishandled looks exactly like one that has not.

This guide describes what the procedure involves. It is written for people who have already decided to do this and want to do it correctly, and it does not argue that anyone should. Most of the compounds people reconstitute at home are not approved for human use anywhere, which is covered on every compound page on this site.

What reconstitution actually is#

Peptides are shipped freeze-dried, a process usually printed on the vial as "lyophilised". The manufacturer froze the peptide in solution and then drew off the water under vacuum, which leaves behind a dry cake or a thin film on the glass. Dry peptide is far more stable than wet peptide, which is why it is sold this way and why the powder tolerates shipping that a solution would not.

Reconstitution reverses that. Adding a sterile liquid dissolves the powder back into solution, at which point the clock starts: a reconstituted peptide degrades in a way the powder does not, and it needs refrigeration from that moment on. The storage and handling post covers what happens to a solution over the following weeks.

The vial is also usually under vacuum. That has a practical consequence during the procedure, and it is worth knowing about before it surprises you.

Bacteriostatic water and sterile water are not the same thing#

Bacteriostatic water is sterile water containing roughly 0.9% benzyl alcohol, which is a preservative. The preservative is what makes a vial multi-dose: it suppresses bacterial growth in the solution across repeated punctures of the stopper, which is why bacteriostatic water carries a conventional in-use limit of about 28 days after first entry.

Sterile water for injection contains no preservative. It is intended for a single use, and a vial reconstituted with it has no defence against anything introduced by a needle. Some peptides are incompatible with benzyl alcohol and are specified for sterile water or for a different diluent entirely, and a few require an acidic diluent because they will not dissolve in plain water at all. Which diluent a given compound needs is a property of that compound, not a matter of preference.

Deciding how much liquid to add#

The amount of diluent is a free choice, and it is the choice that determines everything downstream. It does not change the total quantity of peptide in the vial. It changes the concentration, and therefore how many marks on a syringe a given dose works out to.

• More diluent means a weaker solution. A dose occupies more volume and spans more graduations on the barrel, so a small measurement error costs proportionally less. The trade is that the injected volume is larger.
• Less diluent means a stronger solution. The injected volume is smaller, which is more comfortable, but each graduation on the syringe now carries more peptide and a misread mark costs more.

The usual advice is to add enough that a single dose lands somewhere in the middle of the barrel rather than in the first two or three graduations, because the bottom of an insulin syringe is where measurement is least reliable. The reconstitution calculator does the arithmetic and shows where the dose falls on the barrel, which is faster than working it out on paper.

There is also a ceiling: the vial has to physically hold the liquid. A 2 mL vial will not take 3 mL of water, and most peptide vials are smaller than people expect.

The order of operations#

The sequence below is the standard aseptic procedure. The steps that look fussy are the ones that matter most.

1. Let the vial reach room temperature. Reconstituting a vial straight from the fridge pulls condensation onto cold glass and makes the powder harder to read.
2. Wash your hands, then flip the caps off both vials. The plastic disc comes off; the grey rubber stopper underneath stays where it is. It is designed to be punctured, not removed.
3. Swab both stoppers with a fresh alcohol wipe and let them dry. Alcohol works by evaporation, so a stopper that is still wet has not been disinfected yet. Use one wipe per stopper.
4. Draw the measured diluent into a syringe from the bacteriostatic water vial. A larger-gauge needle draws faster than a fine one, which is one of the reasons the needles guide treats drawing up and injecting as separate jobs.
5. Insert the needle into the peptide vial at an angle and aim the stream at the glass wall. The liquid should run down the side of the vial. Firing it directly onto the powder is the single most common way to damage a peptide, because the force of the jet can break the molecule apart.
6. Let the vacuum do the work. If the vial is still under vacuum it will draw the liquid in on its own. Resist it rather than forcing the plunger.
7. Do not shake it. Swirl the vial slowly, or set it down and let it dissolve by itself, which for most peptides takes a minute or two. Shaking introduces foam and mechanical stress, and both damage peptides.
8. Look at it in good light. A correctly reconstituted vial is clear and colourless, with nothing floating in it.

After it is mixed#

A reconstituted vial goes into the fridge, at 2 to 8 degrees, away from light. It does not go in the freezer: freezing a solution is a second stress on the peptide and is not the same as the freeze-drying the manufacturer did under controlled conditions.

Label the vial with the date it was mixed and the concentration. This sounds like bureaucracy and is the single most useful thing on this page. A month later, an unlabelled vial in a fridge is an unknown quantity of an unknown compound at an unknown age, and the only safe thing to do with it is throw it away.

Signs the vial should not be used#

Some failures are visible and some are not, which is why the visible ones are worth taking seriously.

• Cloudiness or haze in a solution that should be clear.
• Visible particles, strands or flakes, including anything that appeared after the vial had been sitting.
• A film or ring on the glass at the liquid line.
• Foam that does not settle after several minutes.
• A stopper that has been punctured many times, which is a contamination and coring risk rather than a peptide problem.

None of these can be fixed by filtering, re-mixing or waiting. Peptide identity and purity cannot be judged by eye at all, which is a separate problem covered in the purity and testing post.

Frequently asked questions#

Does it matter if some powder sticks to the stopper?

Yes, in the sense that it is peptide you have paid for and will not receive. Tapping the vial before opening it settles most of the powder to the bottom. Once liquid is in, swirling picks up what is on the walls.

How long does the powder last before mixing?

Longer than the solution does, and how much longer depends on the compound and the storage temperature. Dry lyophilised peptide is stable for months to years refrigerated, and that is one reason to mix only the vial you are using.

Can the same water vial be used for several peptide vials?

That is what bacteriostatic water is for, provided the stopper is swabbed before every entry and a fresh needle is used every time. The 28-day in-use limit runs from the first puncture, not from each one.

Does the volume of water need to be exact?

It needs to be known. An extra 0.1 mL is not a safety problem, but it does shift the concentration, and every dose calculated afterwards is wrong by that amount unless the real figure is the one entered into the calculator.