Lyophilised (freeze-dried) peptides are remarkably stable. Removing more than 99% of the water suppresses molecular motion and halts most degradation pathways, which is why unopened vials can be stored refrigerated — or even at room temperature short-term — for extended periods before use.
Reconstitution changes everything. Reintroducing water reactivates every degradation pathway that freeze-drying had paused: hydrolysis, oxidation of methionine and cysteine residues, deamidation of asparagine and glutamine, and aggregation from repeated freeze-thaw cycling. A solution can look perfectly clear while meaningfully losing potency to these processes.
The practical takeaway from the pharmaceutical stability literature: store reconstituted peptides refrigerated at 2–8°C (not frozen, since freeze-thaw cycling itself damages the peptide), out of direct light, and use within the window specified by your preservative — typically 28 days for bacteriostatic-water preparations. Freezing a reconstituted vial "to make it last longer" is usually counterproductive.
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