How to Store Peptides: Before and After Reconstitution

A lyophilised peptide and a reconstituted one are two different storage problems. The dry powder is the stable, long-lived form and tolerates freezing. The solution is fragile, must never be frozen, and starts a countdown the moment water goes in.

Treating them as one thing is the most common storage mistake, and it usually shows up as a vial that was frozen after mixing.

Before reconstitution: the dry powder

Lyophilised peptide is freeze-dried to remove the water that drives most degradation. In that state it is remarkably durable.

• Keep it dry. Lyophilised powder is hygroscopic — it pulls moisture from air, and absorbed water restarts degradation. Keep the stopper sealed.

• Keep it dark. Peptides containing aromatic residues such as tryptophan and phenylalanine are photosensitive.

• Warm before opening. Bring a cold vial to room temperature before breaking the seal, or condensation forms inside it. Published peptide-handling guidance recommends warming lyophilised material in a desiccator for exactly this reason.

After reconstitution: the solution

Once water is added, the rules change completely.

• 2–8 °C, on a shelf. Not the fridge door, which is the warmest part and swings in temperature with every opening.

• Never freeze. Ice crystal formation and the concentration changes during freezing and thawing both stress peptides in solution. Published guidance is explicit about avoiding repeated freeze-thaw cycles.

• Protect from light. Keep it in a box or an opaque container rather than loose on a shelf.

• Do not shake. Agitation shears peptide chains and creates foam that makes accurate draws impossible.

Label the vial

A reconstituted vial with nothing written on it is a vial you will be guessing about in three weeks. At minimum, record:

1. The date it was mixed — the 28-day clock starts here, not at purchase.

2. The resulting concentration in mg/mL, so you never re-derive it from memory.

3. The compound and vial size, if you have more than one in the fridge.

The Peptide Library inventory tracks mix date and concentration per vial, which means the expiry is something you can look up rather than reconstruct.

Signs a vial should be discarded

• Cloudiness or particulate that will not dissolve with gentle swirling.

• Any change in colour.

• Past its beyond-use date, whichever of the two clocks came first.

• A stopper that was left uncovered, or that you are unsure was swabbed.

• Any vial you cannot date with confidence.

Frequently asked questions

Can I freeze a reconstituted vial to make it last longer?

No. Freezing is for the dry powder. Freezing a solution stresses the peptide and does not extend the preservative's rating.

Does bacteriostatic water itself need refrigerating?

No — an unopened bottle is stored at controlled room temperature, 20–25 °C. Refrigeration applies once it has been mixed with peptide. See the bacteriostatic water guide.

How long does lyophilised powder last at room temperature?

Days to weeks without meaningful loss, which is why shipping without cold chain is common. It is a transit tolerance, not a storage plan.