What is a main concern when using metallic hair dyes on previously dyed hair?

The use of metallic hair dyes on previously dyed hair is particularly concerning because these dyes can cause severe damage and hair breakage. Metallic dyes work differently than traditional dyes; they often contain metals such as lead or silver and can react chemically with the residues of previous hair dyes. This reaction can compromise the hair's integrity, leading to brittleness, significant damage, and ultimately breakage.

When hair is already processed with other chemical treatments or dyes, the porous structure can become even more vulnerable to harsh metallic compounds. The strength and elasticity of the hair can diminish, resulting in a high risk of extensive damage. This makes it critical for professionals in cosmetology to assess the current state of the hair and to communicate the risks associated with using metallic dyes on already treated hair.