How long do reconstituted peptides last in the fridge?

"How long will this stock last?" is the question we are asked most often after "can I see the COA". The honest answer is that it depends on the peptide, the diluent, the temperature, how many times the vial is opened, and how sensitive your assay is to a few percent loss. This note gives the working rules most laboratories use, the chemistry behind them, and how to tell when a stock is past its useful life. All Alphex products are for laboratory and in-vitro research only.

The working rules

• Lyophilised, sealed: months to years at -20 C; weeks to months at 2-8 C. This is by far the most stable state, which is why peptides are shipped as powder.
• Reconstituted in bacteriostatic water, refrigerated at 2-8 C: most short research peptides remain within a few percent of nominal for two to four weeks. Beyond that, degradation becomes measurable for sensitive assays.
• Reconstituted in sterile water without preservative: treat as single-session or, at most, a few days refrigerated. There is nothing stopping microbial growth once the seal is broken.
• Frozen aliquots at -20 C: months, provided each aliquot is thawed once. Repeated freeze-thaw is what shortens the life.
• Room temperature in solution: hours to a day or two. Do not leave working solutions on the bench between sessions.

These are conservative planning figures for the compounds in the Alphex catalogue: BPC-157, TB-500, GHK-Cu, Ipamorelin and GLP-3 (RT). Larger or more sensitive peptides sit at the short end of each range.

What actually degrades a peptide in solution

• Hydrolysis. Water slowly breaks peptide bonds; the rate rises with temperature and at extremes of pH. Asp-Pro and Asp-Gly linkages are the classic weak points.
• Deamidation. Asparagine and glutamine side chains convert to acids over time, changing the mass by about 1 Da and often the retention time. TB-500's glutamine is an example.
• Oxidation. Methionine, cysteine, tryptophan and to a lesser extent histidine and tyrosine oxidise in the presence of dissolved oxygen and light. GHK-Cu's histidine-copper centre is one reason it is kept dark.
• Aggregation and adsorption. Peptides stick to each other and to plastic; lipidated peptides especially. This does not change the molecule but it removes it from solution, which for a quantitative assay is the same problem.
• Microbial growth. In unpreserved diluent, contamination introduced during sampling multiplies at fridge temperature slowly and at room temperature quickly. Bacteriostatic water's 0.9% benzyl alcohol suppresses this, which is why it extends usable life.

How to tell a stock has gone

• Visible cloudiness, particles, or a film on the glass: aggregation. Discard.
• Colour change: for GHK-Cu, blue drifting to green or a precipitate means copper loss.
• Assay drift: a standard curve that has shifted, or a positive control that has weakened, with the same nominal concentration.
• If you have access to HPLC, a re-run against the original chromatogram is definitive: new peaks or a smaller main peak mean degradation. Our note on recognising peptide degradation goes into more detail.

How to make a stock last longer

1. Reconstitute in bacteriostatic water if the assay tolerates the preservative.
2. Aliquot immediately into single-use volumes and freeze at -20 C; thaw one at a time. See aliquoting peptide stock solutions.
3. Keep everything dark: amber vials or a box in the fridge.
4. Use low-binding tubes and tips for dilute working solutions.
5. Date the vial at reconstitution and at each thaw. Memory is not a record.

Frequently asked questions

Can I refreeze a thawed aliquot?

You can, but each cycle costs a little activity through aggregation and ice-crystal stress. Aliquot small enough that you never need to.

Does the fridge door matter?

Yes. Door shelves swing several degrees every time the fridge opens. Store stocks at the back of a shelf.

Is a slightly cloudy stock still usable?

Not for quantitative work. Cloudiness means an unknown fraction has left solution.

Alphex research peptides are supplied for laboratory and in-vitro research by qualified researchers only. They are not authorised for human or veterinary use.

Research use only. Alphex products are supplied exclusively for laboratory and in-vitro research by qualified researchers. They are not authorised for human or veterinary use, and nothing in this note is advice on any such use.

Published 10 February 2026. Alphex reviews research notes when the testing method, the catalogue or UK guidance changes.

Alphex Research Team

Alphex Limited, UK research peptide supplier

The Alphex research team writes and maintains these notes for UK laboratories buying research peptides. Every batch Alphex lists is verified by reverse-phase HPLC with a batch-matched Certificate of Analysis. Content covers laboratory and in-vitro research only.