What is the main reason to avoid using metallic dyes on previously chemically treated hair?

Using metallic dyes on previously chemically treated hair can lead to severe hair damage or breakage due to the chemical reactions that can occur when the metallic compounds interact with other chemicals already present in the hair. Chemical treatments such as perms, relaxers, or previous hair coloring often alter the hair's structure and balance, making it vulnerable. When metallic dyes are applied, they can create an incompatible reaction with these existing treatments, causing the hair to weaken and potentially break off. This risk is particularly significant because the hair's integrity is already compromised by prior chemical services, making it essential to avoid such dyes to preserve hair health and prevent irreversible damage.