Peptide storage — fridge, freezer, or neither, and why the answer changes after reconstitution
There are really two storage questions and conflating them is where most of the confusion comes from.
**Lyophilised (powder, unreconstituted).** Freeze-drying removes the water that hydrolysis needs, so the powder is comparatively stable. Cool and dark helps; freezing helps more for long horizons. Room temperature for a limited period is generally tolerated, which is why shipping works at all.
**Reconstituted (in solution).** Now there's water, and two clocks start: chemical degradation, and microbial risk. Refrigeration slows both. This is the state that actually needs care.
**Freezing a reconstituted solution is usually a bad idea**, which surprises people who assume colder is always better. Freeze-thaw cycles cause mechanical stress, concentration changes at the ice interface, and can drive aggregation. A peptide that's fine frozen as a powder may not be fine frozen in solution. If freezing solution is unavoidable, single-use aliquots avoid repeat cycling.
**Light and agitation** matter too and get ignored. Some peptides are photosensitive. Vigorous shaking can denature — which is why the advice is to direct the diluent down the vial wall and swirl rather than shake.
Evidence tier: general peptide/protein formulation chemistry. Compound-specific stability data is much thinner and where it exists it should override any general rule here.