Yes — and the shelf life of a reconstituted peptide is far shorter than its shelf life as a lyophilised powder. Once water is reintroduced, degradation pathways that freeze-drying had suppressed (oxidation, deamidation, hydrolysis, aggregation) start running again, gradually reducing potency even if the solution still looks clear.
Two separate clocks are running simultaneously: the peptide's own chemical stability in solution, and the antimicrobial protection window of the bacteriostatic water used to reconstitute it (governed by USP <51> preservative effectiveness standards, typically validated for 28 days under refrigeration). Whichever runs out first effectively ends the vial's usable life.
A vial that has changed colour, turned cloudy, or developed visible particulate matter should be discarded regardless of how long it's been — that's a visible sign of aggregation or contamination, not just a theoretical potency question. As a general research-community convention, most reconstituted peptides are treated as expired after 28–30 days refrigerated, even if they still look normal.
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