glutathione is a protein synthetase/GSS Protein, Human (His) Redox Regulation by Protein S-Glutathionylation:

Visual signs of degradation Injectable GHK-Cu changes: Clear to cloudy (bacterial growth or precipitation) Color change (bluish, greenish tint from copper oxidation) Particles floating in solution Separation or settling Topical serum changes: Darker color (oxidation) Layer separation Texture changes (thicker or thinner) Crystallization Powder changes: Color darkening Clumping (moisture absorption) Greenish or bluish tint Loss of fine powder consistency Smell and texture changes Off odors: Metallic smell intensifying (some metallic odor is normal for copper peptides) Sour or rancid smell (bacterial contamination or lipid oxidation in serums) Any unusual smell different from when first opened Texture issues: Serum becoming grainy Unusual thickness or thinning Sticky residue Separation that doesn't re-mix Effectiveness loss Signs of reduced potency: Reduced healing speed (injectable) Less visible skin improvement (topical) Need to use more product for same effect Complete lack of expected results When in doubt: Discard and get fresh product

How to Use: Apply evenly to clean, dry skin and massage until absorbed

DOI: Pavlides M, Cleland J, Rahman M, et al

Step both compounds up slowly over the next 16 weeks to reach maintenance

Who Might Benefit From One Over the Other If your skin tolerates retinol well and you've built a stable routine around it, there's no reason to swap