Isotonicity and what happens when you get it wrong

Buffers and pH: the pH of a solution is buffered to maintain stability. The buffer system (phosphate, Tris, or other) is chosen to keep the solution at the pH where the peptide is most stable.

Lyophilisation and reconstitution: the powder is stable for extended periods if kept cool and dry. Once reconstituted, stability decreases because the dissolved peptide is more vulnerable to oxidation and other degradation.

This follows post #15 rather than contradicting it.

Fill volume and headspace: the amount of solution in a vial and the air above it affect how much the solution is exposed to oxygen during storage. Less headspace means less oxygen exposure.

post #24 answers the question as asked. The question underneath it is different.

Preservatives: solutions for multi-dose vials contain preservatives (benzyl alcohol, phenol, or others) to inhibit microbial growth after the vial has been entered. Single-use vials do not need preservatives.

Coming back to post #29, because the follow-up matters more than the original answer.

Antioxidants: compounds that prevent oxidation (which damages peptides) are sometimes included. Methionine oxidation is a common degradation pathway and antioxidants help prevent it.