Cocamidopropyl Betaine.

What it does

It is rarely the main cleanser; instead it boosts and stabilizes foam, builds viscosity, and, crucially, blunts the harshness of stronger surfactants like SLS so a wash cleans well without over-stripping. It is a formulation co-surfactant, not a leave-on skin active.

Benefits

• Boosts and stabilizes foam, giving cleansers a richer, more pleasant lather
• Softens the harshness of stronger surfactants, helping a formula clean without over-cleansing
• Adds antistatic and mild conditioning properties, handy in shampoos and body washes
• Helps control viscosity and is highly biodegradable

Side effects

• Despite its mild reputation, it is a recognized contact allergen, named Allergen of the Year 2004 by the American Contact Dermatitis Society
• Most reactions are attributed to manufacturing impurities (amidoamine and DMAPA) rather than CAPB itself, and are less likely when those impurities are tightly controlled
• On its own it most often does not irritate skin or mucous membranes, but sensitized individuals can react

Who it's for