How do I know if a peptide has degraded?

Visible signs of peptide degradation include cloudiness (which typically indicates aggregation), discoloration (yellowing or browning, which typically indicates oxidation or Maillard-type chemistry), and particulate matter (which typically indicates aggregation or precipitation). Any of these is reason to discard the vial. Unusual smell is also a discard signal.

Degradation can occur without visible signs. Slow hydrolytic cleavage of peptide bonds may reduce potency without producing a change visible to the eye. Oxidation of susceptible residues (methionine, cysteine, tryptophan) may produce partially inactive peptide without visible discoloration. This is why beyond-use dating is important even when the vial looks normal: the peptide may have lost significant activity even without the visible change that would prompt discarding.

The practical policy: check the vial visually before every use. Discard immediately if any visible sign of degradation appears. Discard when the beyond-use date is reached even if no visible change has occurred. When in doubt, discard rather than use — the cost of one discarded vial is much lower than the cost of injecting a degraded product with reduced potency or (in worst-case) a contaminated product with microbial growth.