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There are worries about the acidity of chemical exfoliants lowering the skin's pH and so disrupting retinol's ability to work its anti-ageing, skin-smoothing benefits

Not because GHK-Cu works harder, but because there's more room for visible improvement

Moreover, Kimura et al

Consumer Results 99% agreed it instantly nourished skin with moisture, imparting a smooth, healthy-looking glow 96% agreed skin looks nourished** 95% agreed skin feels deeply moisturized** 95% agreed skin feels immediately hydrated** 94% agreed skin looks and feels healthier** 94% agreed skin feels smoother and more supple** 93% agreed skin looks and feels softer** 93% agreed it improved the overall appearance of skin** 91% agreed skin looks radiant** 88% agreed they love the non-sticky, non-greasy formula** 87% agreed it absorbs quickly into the skin** 81% agreed skin looks and feels more plump** 81% agreed it softens fine lines and wrinkles** 80% agreed skin feels firmer** 79% agreed skin looks firmer** 79% agreed it improves skins elasticity** 78% agreed it dramatically improves skin complexion** 77% agreed it dramatically improves skin texture** 76% agreed skin doesnt feel oily after application** ** In a consumer study of 106 women after 28 days

Bibcode:2019Natur.573..193F