Good question and it actually does matter.

For lyophilized (powder) peptides: the freezer is ideal for long-term storage (-20 degreesC). If you’re going to use them within a few weeks, the fridge is fine. Keep them away from light, the dark back of the fridge shelf, not the door.

Once reconstituted with BAC water: refrigerate and aim to use within 4 weeks. Some people go longer but peptide stability degrades past that point, especially if there are any freeze-thaw cycles.

Also, always use bacteriostatic water (BAC water), not regular sterile water. BAC water contains 0.9% benzyl alcohol which inhibits bacterial growth and extends shelf life significantly.